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