That's conjugation. English uses "eats" for 3rd person singular and "eat" for everything else (I eat, you eat, they eat, we eat).
In Ukranian all 6 forms are different, and the congugation for "to eat" is irregular (sure, it resembles the regular one, but not quite:
- I eat = Я їм
- We eat = Ми їмо
- You(sg.) eat = Ти їси
- You(pl.) eat = Ві їсте
- She eats = Вона їсть
- They eat = Вони їдять
Note that 3rd person forms are, of course, the most used in real life. The "I" form lags a bit behind, too. So, if any, you should remember at least these three: я їм, він їсть, вони їдять. You often say what you or some other "he", "she" and "they" do.