"Therefore I say to you, any sin and blasphemy shall be forgiven men, but blasphemy against the Spirit shall not be forgiven."
It is a attributing some evil to God, or denying Him some good that we should attribute to Him. This case of blasphemy however is a specific one, called "the blasphemy against the Holy Spirit" in Matthew 12:31. In Matthew 12:31-32 the Pharisees, having witnessed irrefutable proof that Jesus was working miracles in the power of the Holy Spirit, claimed instead that the Lord was possessed by the demon "Beelzebub" (Matthew 12:24). Now notice that in Mark 3:30 Jesus is very specific about what exactly they did to commit "the blasphemy against the Holy Spirit"
This blasphemy then has to do with someone accusing Jesus Christ (in person, on earth) of being demon-possessed. It was done by the Jewish religious leaders. There are other ways to blaspheme the Holy Spirit, but this was "THE" ( definite article η in Gk.) blasphemy that was unpardonable. This means that this unpardonable sin against the Holy Spirit cannot be duplicated today because Jesus Christ is not on earth but seated at the right Hand of God!