The MacBook is a great laptop, for the most part. But when you cram so much computer into such a tiny package, you're going to have to make some sacrifices. First on the chopping block: decent speakers.
The BassJump is a subwoofer for the MacBook, basically turning the laptop's built-in speakers into tweeters and handling the bass duties itself. It uses software to separate everything, making it more versatile than if you just plugged a normal subwoofer in.
Of course, you could also get a dedicated set of good speakers, but they wouldn't look like they belonged next to your MacBook, would they?