In dynamics music notation, pianississimo is abbreviated 'ppp' and indicates that the music from that point on should be as soft as possible. The word means "very very quietly" in Italian.
When added to the staff, it looks something like this (complete with eigth notes):
/\
---| /----------------------------------------------------
|/
---/---------------*---------*---------*---------*--------
/| | | | |
-/-|/\--------*---|-----*---|-----*---|-----*---|---------
| | | | | | | | | | |
|--|--|------|----|----|----|----|----|----|----|---------
\ | | |____|____|____|____|____|____|____|
--\|_/----------------------------------------------------
|
\| ppp
There is also a rarely-seen 'pppp' marking, which would be played even more softly than pianississimo.