Più
Italian for "more": strengthens a preceding direction, for example più allegro = faster, più forte = louder.
What it means
Più (Italian for "more") is a word that strengthens or intensifies a performance direction. It stands before a tempo, dynamic or expression marking and asks for an increase of that quality compared with the previous state. Thus più allegro means "livelier" or "faster", più forte "louder", più dolce "sweeter, softer".
Musical use
The word appears in many compound instructions, for example più mosso (more moving), più lento (slower) or più piano (quieter). Its counterpart is meno ("less"). In the score, più always refers to the previous state and not to a fixed value: più forte after a mezzo-forte sounds different from after a forte.
Più should be distinguished from molto ("very"), which intensifies absolutely without referring to the previous state, and from poco ("a little"), which changes only slightly.
Performance practice
When you read più, first check how it was before and increase from there. It would be a mistake to execute più at maximum strength at once; better to increase clearly but in a controlled way.