Redneck is a slur used to describe Whites in a derogatory way. Previously referred only to the rural prejudice whites, mostly farmers, who have reddish necks (or a "farmer's tan"). However, its usage has become a lot looser and now includes any racist white. See: Peckerwood
The word Redneck is a racial slur that has been used historically to degrade and dehumanize whites. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to whites. If you are unsure about whether or not it is appropriate to use the word Redneck, it is best to err on the side of caution and avoid using it in any situation.