It's just how the word looks like in Polish. So far this is just the Nominative, we called it "pomidor" when this vegetable/fruit was being named, and that's it.

"Pomidora" will be the Genitive form and also the Accusative, although there will be two options for its Accusative. You'll encounter a discussion about that later, let's leave it for now.

We just don't have articles.