A canteen is a store that sells food and drink at an institution like a camp, college, or military base. A canteen can also be a small container used to carry water to drink.
If you’re at summer camp, you can buy snacks at the canteen. This kind of canteen is a little shop. At the canteen, you can buy little things like potato chips or a can opener. A canteen is like a convenience store. The other kind of canteen is good if you’re going on a hike — fill it up with water and put it in your backpack, since there probably aren’t any of those other kinds of canteens out in the woods.