In the tradition of boastful, macho construction workers, it's actually any plank with a 1.5" by 3.5" cross-section. When he tells you it adds up to six inches, you'll know it's more like five.
All joking aside, this is your standard wooden board, used for all sorts of things, available in pine, cedar, or other woods, salt treated, kiln dried, or plain, and a variety of lengths. Fortunately when you ask for an eight foot long two by four, you don't get seven and a half feet.
And now, the only (and therefore, worst) joke about two by fours, properly told in a redneck accent:
Q: What's a two b' four?
A: T'play inna
polka band.