It’s a great question because the word heresy sometimes is thrown around by people.
Some people use the word heresy simply to mean any error or a fairly serious error in theology. But classically the word heresy was used to describe those theological errors so serious that it would deprive one of salvation. I think we ought to be using heresy more in that sense.
Error is error. Error can be serious. Error can be small. Error is always bad, and to be avoided. But there are some errors that are so huge that they really are cutting us off from God, because we have so misunderstood Him and His truth. And that’s what heresy classically was used for in the church.