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