Tiny hummingbirds eat mostly nectar, which is a sugary liquid made by flowers.
Ones with really long, curved bills feed only on the nectar of one kind of flower shaped the same way as their bills. Others eat from all sorts of flowers, or special feeders people put out. A lot of people think hummingbirds suck up the sweet treats. That's not true. They lap up the liquid with really long, extendible, tube-like tongues, a lot like the way a kitty cat laps up milk.