In music notation, sforzando-piano is abbreviated "sfp" and is used like other loudness markings. It indicates that the music after that point should be suddenly piano in loudness. It means "forcefully quiet" in Italian.
When added to the staff, it looks something like this (complete with eigth notes and an initial mezzoforte marking):
/\
---| /----------------------------------------------------
|/
---/---------------*---------*---------*---------*--------
/| | | | |
-/-|/\--------*---|-----*---|-----*---|-----*---|---------
| | | | | | | | | | |
|--|--|------|----|----|----|----|----|----|----|---------
\ | | |____|____|____|____|____|____|____|
--\|_/----------------------------------------------------
|
\| mf sfp