Same as "you should", though it can be use for any subject (I, he, she, we, it, they). Implies advice or warning. Common in the southern US, this phrase has been shortened down from:
You would
be better off if you...
You would be better to ...
You'd be better...
You'd better...
You better...
Sometimes as a
threat, a person will mistakingly say "you had better...", but the uncontracted version is "would" not "had".
You better get started on that paper if it's gonna be done by Monday.
You better be in this house when the
street lights come on.
You better put them trash cans up on the porch,
so's the dogs ain't
gettin in 'em.