Learn Croatian: Gost zove konobara i vozača. - The guest is calling the waiter and the driver.

They are the direct objects of the sentence, meaning they receive the action of being called. In Croatian, when a masculine noun refers to a living being (like a waiter or driver) and acts as a direct object, it takes an -a at the end. Their basic dictionary forms are simply konobar and vozač.

Why doesn't Gost have an -a at the end? Isn't a guest a living being too?

Yes, a guest is definitely a living being! However, Gost is the subject of the sentence (the person doing the calling), so it stays in its basic dictionary form. We only add the -a to animate masculine nouns when they are the direct object receiving the action.

Does zove mean "calls" or "is calling"?

It means both! Croatian does not have a separate continuous tense like English does. Depending on the situation, zove naturally translates to either "calls" or "is calling".

What is the mark over the letter č in vozača?

That mark makes the letter č sound like the "ch" in the English word "chalk". A plain c in Croatian always sounds like "ts" (as in "cats"), so this mark is important for getting the pronunciation right.