Huck considers civilization hypocritical. It means that people tell him what to do, and what they say is right and wrong is not necessarily what he considers right and wrong. It means uncomfortable clothes and rules. It is the opposite of freedom.
At the end of the book, Huck comments on why he left.
But I reckon I got to light out for the territory ahead of the rest, because Aunt Sally she's going to adopt me and sivilize me, and I can't stand it. I been there before. (ch 43)
This is a common motif throughout the book—the contrast between civilization and humanity. If civilization means confining clothes, boring school, and stifling religion, Huck can do without it.
Although Huck cannot stand his repulsive and abusive father, he still prefers being able to fish, eat stew, wear comfortable clothes, and sleep outside to being "cramped up" with the widow (ch 6). Huck has grown up wild, and since most people cannot explain why the rules of civilization exist, Huck sees no reason to follow them.