In dynamics music notation, pianissimo is abbreviated 'pp' and indicates that the music from that point on should be very quiet. The word simply means "very quiet" in Italian.

When added to the staff, it looks something like this (complete with eigth notes):

    /\                                                     
---| /----------------------------------------------------
   |/                                                      
---/---------------*---------*---------*---------*--------
  /|              |         |         |         |         
-/-|/\--------*---|-----*---|-----*---|-----*---|---------
|  |  |      |    |    |    |    |    |    |    |         
|--|--|------|----|----|----|----|----|----|----|---------
 \ |  |      |____|____|____|____|____|____|____|         
--\|_/----------------------------------------------------
   |                                                       
  \|       pp

Pi`a*nis"si*mo (?), a.[It., superl. of piano.] Mus.

Very soft; -- a direction to execute a passage as softly as possible. (Abbrev. pp.)

© Webster 1913.