(1) A barf bag, in case you get motion sickness while hiking.
A barf bag is essentially where you put discarded yum-yums (already-eaten food). Or yum-yums you don't want anymore, or need, or like. Or have decided are actually toxic waste. Or what your body has decided, without your conscious input, is toxic waste, or wretched excess.
Anyhow, need for a barf bag is probably a strong hint that hiking isn't for you. Better not to even think about backpacking, at least until the nausea passes.
(2) A bear bag. You score bonus critter points if you sleep directly under it. If you don't know what a bear bag is, you'll probably find out sometime, when you wake up during the night to find your tent full of big furry things ripping your pack to bits, looking for yum-yums.
Good luck with that, Bud.
(3) A garbage bag. For leftovers, anything that can't be eaten, or what no one wants to even think about eating, but that something non-human might eat (or fight you for). The garbage bag concept is especially useful for a gaggle of boy scouts who haven't learned the rule to eat all you cook and cook all you eat, or die trying.
This sort of bag of glop is great as an implement of punishment, by making anyone who breaks the rules or is just too sleepy to get up on time carry and look after it, thus ruining for life another person's attitude toward the outdoors and the whole idea of spending time with others having "fun".
(4) A bag for snacks, or a bag of snacks. But if you're a rat, it can be a garbage bag too.
Who cares, really?
Dinner-plate scrapings are as good as anything else. Especially if you're a rat. Or a grizzly bear.
(5) One of those plastic bags that you can roast critters in, and when used as a garbage bag, doubles as that critter's last resting place. I.e., a crinkly, see-through casket. (You can watch the dead eyes follow you around as you try to get away, and so on.)