What does it mean?

Barbara,
You ask a good question, the answer to which is uncertain, but which believers often answer with unsubstantiated pontifications. Let me begin by quoting the Didache (95 AD), the earliest clarification outside the New Testament. The Didache (French shorthand for "The Teaching of the Twelve Apostles") was written in the era of the composition of New Testament writings. It has access to one of the sources used by Matthew. Here is what it says about your question:

Chapter 11. Concerning Teachers, Apostles, and Prophets. Whosoever, therefore, comes and teaches you all these things that have been said before, receive him. But if the teacher himself turns and teaches another doctrine to the destruction of this, hear him not. But if he teaches so as to increase righteousness and the knowledge of the Lord, receive him as the Lord. But concerning the apostles and prophets, act according to the decree of the Gospel. Let every apostle who comes to you be received as the Lord. But he shall not remain more than one day; or two days, if there's a need. But if he remains three days, he is a false prophet. And when the apostle goes away, let him take nothing but bread until he lodges. If he asks for money, he is a false prophet. And every prophet who speaks in the Spirit you shall neither try nor judge; for every sin shall be forgiven, but this sin shall not be forgiven. But not every one who speaks in the Spirit is a prophet; but only if he holds the ways of the Lord. Therefore from their ways shall the false prophet and the prophet be known. And every prophet who orders a meal in the Spirit does not eat it, unless he is indeed a false prophet. And every prophet who teaches the truth, but does not do what he teaches, is a false prophet. And every prophet, proved true, working unto the mystery of the Church in the world, yet not teaching others to do what he himself does, shall not be judged among you, for with God he has his judgment; for so did also the ancient prophets. But whoever says in the Spirit, Give me money, or something else, you shall not listen to him. But if he tells you to give for others' sake who are in need, let no one judge him.

In other words, an authentic prophetic word is self-authenticating because the Holy Spirit confirms its truth by an inner witness. So if the Spirit confirms a prophetic word to your spirit and you reject what you know to be God's word because you don't want to apply it to your life, that sin is unpardonable in the sense that the Holy Spirit is grieved and will no longer speak to you or act in your life. So I doubt that anyone who wants to repent and have a personal relationship with God has committed the unpardonable sin.

The relevant New Testament texts are Mark 3:28-29, Matthew 12:31-32, and 1 John 5:16. Jesus applies the saying to those who have been exposed to the liberating compassion of His exorcisms and yet persist in attributing His exorcisms to demonic power.

 

Only could be done by the Pharisees themselves, as their sin was to see jesus do His miracles, hear His words, and while lookinmg right at Him, call Him the spawn of satan, as a tool of the Devil!