Life on the raft represents freedom for both Jim and Huck. On dry land, they feel restricted in their movements—especially Jim, who, as a slave, has no freedom in so-called civilized society. As for Huck, he's always been a child of nature, preferring to spend his days wandering around fields and woods, going fishing, or enjoying a nice swim on a hot summer's day. This is his natural habitat, the place where he feels most comfortable, and the place where he can be himself.
Traveling down the Mississippi on a raft means that Jim and Huck never have to spend too much time in any one place, and that's how they like it. To both of them, freedom means being able to control your own destiny and that, in turn, means choosing where you live and for how long. On dry land, this wouldn't be possible. But out there on the raft, in the freedom of the open air, Jim and Huck can live life on their own terms in a way that would be unthinkable back in society.