"Wherefore I say unto you, All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Ghost shall not be forgiven unto men. And whosoever speaketh a word against the Son of man, it shall be forgiven him: but whosoever speaketh against the Holy Ghost, it shall not be forgiven him, neither in this world, neither in the world to come."
Blasphemy against the Holy Spirit is not a single action, and it is not accidental or casual. So if you're worried you may be guilty of it, you are not. Rather, it is a persistent and hardened rejection, a deliberate and often malicious denial of God's power and the Holy Spirit's influence.
Prior to these verses, the Pharisees had attributed Jesus's miracles to demonic powers. This is presented as an example of this type of blasphemy. Despite Jesus's meek and compassionate nature on display, these Pharisees had fallen into a pattern of behavior characterized by a hardened heart and a rejection of God's truth.
The reason this sin is unforgivable is because the guilty party is not seeking repentance. They are in a state of spiritual denial. God's grace and forgiveness are available to those who repent and seek Him.