Learn Portuguese (Portugal): O gato vai para cima da mesa.

Cima just means "top" or "up". Because the cat is moving towards the table, we need the preposition para (to/towards) to show the direction. Together, para cima means the cat is moving "upwards" or "up onto" something.

Where does the word da come from in this sentence?

Da is a mandatory contraction of the preposition de (of) and the feminine word for "the", which is a. In Portuguese, phrases about location like "top" always take de to connect to the object. So, "top of the table" is cima de + a mesa, which shrinks into cima da mesa.

What verb is vai from?

It comes from the verb ir (to go), which is one of the most common and irregular verbs in Portuguese. Vai is the form used for he, she, or it—in this case, o gato.

Could I use na instead of para cima da?

Not in this case, because there is movement involved. If the cat was already sitting on the table, you would say the cat is na mesa (on the table). But because the cat is actively moving onto it, Portuguese uses para cima da (literally "towards the top of the") to show that change in location.