Ponder refers to thinking in the opponents time:
To use the opponents time, to continue searching the tree and store the information in a large lookup table for later use.
This will save time when its our turn.
Requires transposition tables.
While the opponent is looking for a move, instead of sitting idle, guess their most likely move and start analyzing the resulting position.
If we guess right, we have a head start searching;
if we guess wrong, we have wasted no time and will have at least somewhat populated the transposition tables.
More useful against humans, and engines running on separate machines.