The person. "mange": first or third person (and second person if this is an order, imperative mood, like "mange !"). "manges": second person. It's the conjugation of the verbs from the first group: (I) je mange, (you) tu manges, (he/she) il/elle/on mange, (we) nous mangeons, (you) vous mangez, (they) ils/elles mangent.