"She sings softly."
Translation:Lei canta piano.
It's all related, my friend. I believe it originally meant "flat", which diversified into "plain" (i.e. not with jutting extra stuff), "soft" (i.e. not with prickly thorns), "plane" (turning it to a noun), "floor" (i.e. a level element of a building), "plan" (because paper is flat, I'd say), and "piano" (no idea there). As an adverb, it means "slowly" and "carefully", probably evolved from "plain" and "soft".
Piano can also mean "quiet", and the Italian name for the musical instrument is "pianoforte", which means "quiet and loud". The thing is, that the closest predecessor to the piano was another instrument (I can't remember its English name), which could be played with one level of loudness only, and when the piano was invented, it was "a breakthrough" in music, because loudness changes could also be a part of music now :) hope it's interesting