Homebrew is creating a game from scratch usually for some sort of console. Think of it as a non-commercial game built for a console.
In my case, I've been looking at creating my own games for the GBA, and if can't, I should at least get some good information on ASM if I were to return hacking.
Game making's not for me, mainly because I don't want to pay for RPG Maker.
But I've been looking at GBA Homebrew in spare time, really just out of boredom. Although it would be much better if all the good documents weren't 400 pages long, but I guess that's what makes them good, because they go through close to every little detail. :\
Oh well, what are you gonna do? If I want to go through with it and learn, I've really got no choice.