"It's a bunch of b sides from their singles."
B-sides are usually songs that do not appear on the actually album that the A-side appears on, ocassionally it does. The b-side song was usually determined not to fit in with the overall sound of the album or it was considered not good enough to be on the album or just plain leftover tracks, so hence it's usage as a b-side. B-sides have often gotten a reputation of being filler and sub-par, even though a good majority of the time b-sides are better than a lot of the album tracks, and in some cases, better than the A-side.
"Heartbreaker" is the b-side to P!nk's 2006 single "Stupid Girls."