When a musician performs something pianissimo, she plays very softly. If you’re playing a pianissimo piece on the piano, your fingers will be gentle on the keys.
You can use this word as an adverb or an adjective — in either case, it’s a specific musical direction regarding the dynamics (or loudness) of a piece of music. On sheet music, pianissimo is often marked pp, to distinguish it from piano, which is simply “soft,” rather than “very soft,” and is usually marked with one p. There is, in fact, an even quieter designation called pianississimo, that’s “very very soft.”
a direction in music; to be played very softly
chiefly a direction or description in music; very soft
(music) low loudness