In music notation, a double bar-line is two vertical lines close to each other drawn through the staff or completely through the grand staff, in the same manner as a bar-line. A double bar-line indicates the end of a section. When the second line is thicker than the first, it indicates the end of the piece.

It looks something like this (complete with staff, treble clef, key signature, time signature, and a quarter note scale):

    /\                                                     
---| /----------------------|--|---------------*---||
   |/                   |   |  |           *  |    ||
---/----------------|---|---|--|-------*--|---|----||
  /|       4    |   |   |   |  |   *  |   |   |    ||
-/-|/\--b-------|---|---|--*---|--|---|---|---|----||
|  |  |    4    |   |  *       |  |   |   |        ||
|--|--|---------|--*-----------|--|---|------------||
 \ |  |        *               |  |                ||
--\|_/-------------------------|-------------------||
   |                                                       
  \|