If you see piu in your sheet music, you know that you have to do more of whatever comes after it — more loudly if it's piu forte, more quietly if it's piu piano, and more quickly if it's piu allegro.
Più is Italian for "more." It can be part of all sorts of instructions in music. In addition to those mentioned above, you might see it in più lento, "more slowly," or più espressivo, "more expressively." If it's after poco, which means "a little," then you need to do just a little more of whatever the main instruction is; for example, poco più animato means "a little more lively."