My pastor teaches and preaches that after the tribulation those that heard and did not believe the gospel will still have a chance to be saved.  Is this what some refer to as second chance salvation?

I am not sure about the meaning of "second-chance" salvation. Most of us have numerous chances to trust in the Lord before we accept His offer of eternal life. However, I would like to make a some comments about salvation in the tribulation.

• It is true that souls will be saved during the tribulation. Revelation 5:9And they sung a new song, saying, Thou art worthy to take the book, and to open the seals thereof: for thou wast slain, and hast redeemed us to God by thy blood out of every kindred, and tongue, and people, and nation;
  See All... speaks of multitudes that were redeemed to God by the blood of Christ "out of every kindred, and tongue, and people, and nation." Revelation 7:14And I said unto him, Sir, thou knowest. And he said to me, These are they which came out of great tribulation, and have washed their robes, and made them white in the blood of the Lamb.
  See All... refers to those who "came out of great tribulation, and have washed their robes, and made them white in the blood of the Lamb." It sounds as if there may be a revival of sorts during this time. God will show His light even in the darkest of times.
• Yet, we should not think that salvation comes as easily as it does today. For one thing, the taking of the mark of the beast evidently condemns the one who takes it to certain judgment. In Revelation 20:4And I saw thrones, and they sat upon them, and judgment was given unto them: and I saw the souls of them that were beheaded for the witness of Jesus, and for the word of God, and which had not worshipped the beast, neither his image, neither had received his mark upon their foreheads, or in their hands; and they lived and reigned with Christ a thousand years.
  See All..., those who are saved out of the tribulation are identified as those who "had not worshipped the beast, neither his image, neither had received his mark upon their foreheads, or in their hands." Those who have received the mark seem to have sealed their fate to the same degree as those who die without Jesus Christ. They have rejected the Lord and He absolutely rejects them. Though they may live for a time, their condemnation is certain. The following passage is very clear:

Revelation 14:9-11 [9] And the third angel followed them, saying with a loud voice, If any man worship the beast and his image, and receive his mark in his forehead, or in his hand, [10] The same shall drink of the wine of the wrath of God, which is poured out without mixture into the cup of his indignation; and he shall be tormented with fire and brimstone in the presence of the holy angels, and in the presence of the Lamb: [11] And the smoke of their torment ascendeth up for ever and ever: and they have no rest day nor night, who worship the beast and his image, and whosoever receiveth the mark of his name.
See All... And the third angel followed them, saying with a loud voice, If any man worship the beast and his image, and receive his mark in his forehead, or in his hand, The same shall drink of the wine of the wrath of God, which is poured out without mixture into the cup of his indignation; and he shall be tormented with fire and brimstone in the presence of the holy angels, and in the presence of the Lamb: And the smoke of their torment ascendeth up for ever and ever: and they have no rest day nor night, who worship the beast and his image, and whosoever receiveth the mark of his name.

• Another exceptional characteristic of the tribulation is the one to which you refer in your question. I believe that this question is answered in 2 Thessalonians, chapter two. Please consider the following verses:

2 Thessalonians 2:8-12 [8] And then shall that Wicked be revealed, whom the Lord shall consume with the spirit of his mouth, and shall destroy with the brightness of his coming: [9] Even him, whose coming is after the working of Satan with all power and signs and lying wonders, [10] And with all deceivableness of unrighteousness in them that perish; because they received not the love of the truth, that they might be saved. [11] And for this cause God shall send them strong delusion, that they should believe a lie: [12] That they all might be damned who believed not the truth, but had pleasure in unrighteousness.
See All... And then shall that Wicked be revealed, whom the Lord shall consume with the spirit of his mouth, and shall destroy with the brightness of his coming: Even him, whose coming is after the working of Satan with all power and signs and lying wonders, And with all deceivableness of unrighteousness in them that perish; because they received not the love of the truth, that they might be saved. And for this cause God shall send them strong delusion, that they should believe a lie: That they all might be damned who believed not the truth, but had pleasure in unrighteousness.

This passage states that God will send strong delusion to those who had not believed the truth but had pleasure in unrighteousness so that they might believe a lie and be damned. Because of this passage, I believe that those who hear and understand the gospel message and yet blatantly reject it in this age of grace will not believe in Christ during the tribulation. They have rejected the clear and wonderful offer of God's grace and so God will reject them.

I am aware that certain Bible teachers are presently turning away from this teaching, but I have never seen an explanation of 2 Thessalonians 2:11-12 [11] And for this cause God shall send them strong delusion, that they should believe a lie: [12] That they all might be damned who believed not the truth, but had pleasure in unrighteousness.
See All... that convinced me otherwise. I believe that the multitudes that are saved during the tribulation will be from those who never had a clear understanding of the gospel before the rapture. In one sense, God will give them a second chance when He sends the 144,000 around the globe as witnesses to the love of Christ for their souls.