There are a lot of people who are willing to give windy explanations but the truth is that no one really knows exactly what the saying originally meant, and it’s the same with a lot of Jesus’ sayings. Some of the meanings are obscure or lost, and pontificators just insert their own theological biases or assumptions into the sayings.
It’s not only unclear what Mark intended by including the saying (and Matthew and Luke both used Mark as an extensive source for their own Gospels), it’s by no means certain that Mark took the same meaning from it that was intended by Jesus (or whoever the author of the saying was).
Also, it has to be remembered that the concept of a “Trinity” did not yet exist for either Jesus or Mark. Neither of them knew of any other Divine “persons.” There was God, and that was it. Trinitarian theology wasn’t invented until much later. It is not actually found in the Bible.
One possible explanation would be to note that Mark has Jesus say that whoever blasphemes the “Son of man” will be forgiven and to bear in mind that the phrase “son of man” (actually “son of Adam”) was a figurative way to refer to all human beings. Therefore, whoever blasphemes against human beings will be forgiven, but whoever blasphemes against God will not.
Of course, Mark interpreted “son of Adam” Messianically, but Mark was an adoptionist. He describes the Holy Spirit as “descending” upon Jesus at his baptism and Jesus becoming the figurative (“adopted”) “son of God” at that point. It may be that Mark was attepting to draw a distinction between “blaspheming” Jesus as a human “son of Adam” and blaspheming or thwarting the divine spirit and will which drove his mission.
There are other meanings for “Holy Spirit” in ancient Judaism as well as some other early forms of Christianity (like gnosticism). I don’t want to get into a bunch of alternative explanations but any interpretation of “Holy Spirit” from a trinitarian perspective would be historically anachronistic on the part of either Jesus or the author of Mark.
Oh…and there are four canonical Gospels, not five.