A tune is what you whistle when you're feeling cheerful. It's a melody or a specific sequence of musical notes.
Before playing a tune on your guitar, you might want to tune it — in other words, adjust its pitch so it sounds right. As a verb, there are several meanings of tune. You can tune a radio, or adjust the dial until you can heard a certain station, and you can tune a car, or adjust its engine so it works correctly. Tune comes from tone and its Proto-Indo-European root, ten-, "to stretch."