Redneck humor was made famous by comedian Jeff Foxworthy. It makes fun of the stereotypical southerner. You don't have to be a redneck to get it, but I think it helps if you live in the south as you understand it a little better.
Redneck humor is usually considered as one of two things. Humor that depicts rednecks in a funny way, or humor that rednecks make up, usually regarding their lifestyle or job. You do not have to be a redneck to get it.