In music notation, a mordent is a type of ornamentation drawn as a short stylized zig-zag positioned above the staff directly over the note it affects. The zig-zag may have a vertical line through the middle, indicating that it is a "lower mordent" instead of an "upper mordent".
The effect is to play the note as a sort of very short trill, playing the specified note quickly, then one step up (upper mordent) or down (lower mordent) quickly, then the specified note for the remaining duration. The mordent begins on the beat and "steals" time from the duration of the affected note.
On the staff, it looks something like this (complete with time signature):
/\ /\/ /X/
---| /---------------------------------|-------------------
|/ |
---/-------------*---------------------|-------------------
/| 4 | * * |
-/-|/\----------|----|-----|------*----|-------------------
| | | 4 | | | | |
|--|--|---------|----|-----|-----|-----|-------------------
\ | | | | | |
--\|_/---------------------------|-----|-------------------
|
\|
Played as:
/\
---| /------------------------------------|---------------
|/ * |
---/-------------*|--*.-------------------|---------------
/| 4 | | | * |
-/-|/\----------|_|-|----|-----*---*.-----|---------------
| | | 4 |_| | | | *| * |
|--|--|---------|_|_|----|----|_|-|---|---|---------------
\ | | | |_| | | |
--\|_/------------------------|_|_|---|---|---------------
| |
\|