In music notation, a breath mark is a type of articulation represented by a curved apostrophe above the staff. It indicates that the user should take a breath at that point. (Obviously, this doesn't apply to stringed or keyboard music.) It is usually added in as a suggestion by the composer or editor to indicate that the musician should take a breath here rather than at a later point when the flow of the music would be more awkwardly interrupted.
It looks something like this (complete with time signature, sixteenth notes, dotted half note and whole note):
/\ /
---| /------------------------------|---------|------------
|/ ____|___ |
---/----------*---*---*---*----/----|---\-----|------------
/| 4 *| *| *| *| O. | O |
-/-|/\-----|-|-|-|-|-|-|-|---|------|---------|------------
| | | 4 | | | | | | | | | | |
|--|--|----|_|_|_|_|_|_|_|---|------|---------|------------
\ | | |_|_|_|_|_|_|_| | | |
--\|_/------------------------------|---------|------------
|
\|