What is blaspheming the Holy Spirit?

Whoever speaks against the Holy Spirit will NOT be forgiven.

Isn't that a bit harsh?

Let's check the context. Matthew 12:

22 Then a demon-oppressed man who was blind and mute was brought to him, and he healed him, so that the man spoke and saw. 23 And all the people were amazed, and said, “Can this be the Son of David?” 24 But when the Pharisees heard it, they said, “It is only by Beelzebul, the prince of demons, that this man casts out demons.”

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These Pharisees have hardened their hearts so much that they stubbornly attribute the work of the Holy Spirit to the evil spirit.

31Therefore I tell you, every sin and blasphemy will be forgiven people, but the blasphemy against the Spirit will not be forgiven.

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Stubbornly calling the Holy Spirit evil will not be forgiven.

32 And whoever speaks a word against the Son of Man will be forgiven, but whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come.

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If a person keeps calling the Holy Spirit evil, he could not have been born of the Spirit, the Spirit does not dwell in him, and he has not been born again. In fact, he is condemning himself.

Mark 3:

28“Truly, I say to you, all sins will be forgiven the children of man, and whatever blasphemies they utter, 29but whoever blasphemes against the Holy Spirit never has forgiveness, but is guilty of an eternal sin”— 30for they were saying, “He has an unclean spirit.”

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They stubbornly accuse the Holy Spirit in Jesus as unclean. Persisting in calling the Holy Spirit evil is a specific case of blasphemy against the Spirit. For the general case, see What is the unforgivable sin?.