"I see the food."
Translation:אני רואָה את האוכל.
We use את when BOTH these things are true:
1) The word is the direct object of the sentence. In other words: it is receiving the action of the verb. It is being seen, eaten, read, created, etc.
2) This word is ALSO definite/specific. It is not just food in general, but specifically THE food being eaten. Not just a book (any book in general), but specifically THE book being read. Proper nouns are also definite and require את. Duo will introduce other instances later in the tree :-)