Dynamics markings are notation symbols that show how loudly or softly to play. Forte means loud and piano means soft; that is the basic idea.
How to use them
They are used to share the large contours of a piece. In performance you express them by changing not only the volume but also the tone and the attack.
How they differ from related terms
Separately from the instrument called the piano, p is used as a marking meaning to play softly. Crescendo is an instruction to get gradually louder.
Treating the symbols as roles within the piece rather than as mechanical volume values sounds more natural.
