Mangiare is the verb and when conjugated for the Io form (I) it becomes Mangio, for Tu form (You) becomes Mangi, for Lui/Lei (He/She) it becomes Mangia. Now they all mean eat, I eat, You eat, He/She eats.
So the fact that its not eats in this case is because you don't say "You eats" its just "You eat" but if the sentence was "Lui Mangia la Mela" then it would be "He eats the apple".
I use Google Translate to help with the pronunciation of sentences, but when I typed in the phrase "you eat the apple" it said "si mangia la mela" with a secondary option of "mangi la mela." Is this just a preferred way of saying it or maybe something that differs by location and age?
Well, as a native speaker I can tell you that "si mangia la mela" is not a wrong translation if you insert it in a proper context, such as a hypothetical clause... it just means another thing, that is to say the impersonal form "one eats the apple", which in Italian is "si mangia una mela" (with the so-called "impersonal/passivating 'si'": the standard way of rendering an impersonal phrase of this kind) or it is rendered with the second person singular (you) - as in English -, but only in informal speech. For example (I'm sorry for the stupid sentence): 1. EN: "If you eat an apple a day, you will have great benefits". ITA: "Se mangi una mela al giorno, ne trarrai grandi benefici" (informal way of rendering some impersonal phrases); 2. EN: "If one eats an apple a day, he will have great benefits". ITA: "Se si mangia una mela al giorno, se ne trarranno grandi benefici" (standard way of rendering an impersonal phrase).