In music notation, a double natural is a symbol that nullifies a double sharp or double flat. It looks like two natural symbols side-by-side (a pair of percent signs is probably the closest ASCII approximation).
When a note is modified by a double sharp or a double flat as an accidental, it continues for the rest of the measure. In order to cancel this change, the note that was changed is followed by a double natural symbol, which like other accidentals continues for the rest of the measure.
If the composer wants to reduce a double sharp to a sharp, or a double flat to a flat, they would use a single natural symbol followed by a sharp or flat symbol (%#, %b). A double natural "resets" the note by two half steps; a natural with a sharp or flat reset it by only half a step.
A double natural looks something like this, when used as an accidental with a quarter note to cancel a double sharp (complete with staff and key signature):
/\
----| /--#-------------------------------------------------
|/
----/--------#-----------*---------------------------------
/| # x * | %% *
--/-|/\-------------|---|----*---|-------------------------
| | | | | | |
-|--|--|---#--------|---|---|----|-------------------------
\ | | | | |
---\|_/---------------------|------------------------------
|
\|