In dynamics music notation, mezzopiano is abbreviated 'mp' and indicates that the music from that point on should be somewhat quiet. The word means "medium quiet" in Italian.
When added to the staff, it looks something like this (complete with eigth notes):
/\
---| /----------------------------------------------------
|/
---/---------------*---------*---------*---------*--------
/| | | | |
-/-|/\--------*---|-----*---|-----*---|-----*---|---------
| | | | | | | | | | |
|--|--|------|----|----|----|----|----|----|----|---------
\ | | |____|____|____|____|____|____|____|
--\|_/----------------------------------------------------
|
\| mp