In music, a "phrase" typically refers to a musical idea or a short musical segment that expresses a complete thought, similar to a sentence in language. It is often characterized by its melodic contour and is usually made up of several measures.
The other options you mentioned can refer to different musical concepts:
- A place of rest in music might refer to a "cadence," which creates a sense of closure.
- A short recurring musical idea is often called a "motif."
- A note to the performer to play softly would be indicated by the term "piano" or the symbol "p" in sheet music.
So, in summary, the phrase you are looking for in relation to your question is primarily a "musical idea."