If you throw it in the trash, you can call it garbage. Likewise, if something is so terrible you wish you could throw it in the trash, like a really bad movie, you can also call it garbage.
Everything in your kitchen trash is garbage — essentially, it's anything that's useless. Garbage can include vegetable scraps, wadded-up paper, or even spoken gibberish. The origin of the word garbage is a little uncertain, though in the 15th century it was used to mean literally "giblets of a fowl," or the extra parts that weren't considered food.