If you have an ally, you have someone who is on your side, like a more experienced teammate who is your ally in convincing the coach to give you more playing time.
Ally comes from the Latin word alligare, meaning "to bind to," like nations who are allies in wartime — they will act together, and protect one another. You can also use ally as a verb, meaning "join forces with." For example, you might ally yourself with influential people to advance your career. As a noun, pronounce ally like this: "AL-eye." As a verb, it's "uh-LIE."