The flash pan is the part in matchlocks, wheellocks, and flintlocks that contains the powder, which sets off the main charge (powder in the barrel) after being ignited by sparks that were produced by the outer mechanics of the firearm.
The matchlock used a slow-burning wick which is pressed into the flash pan which set off the powder. Flash pan technology was obsolete when the caplock mechanism was invented, which used a percussion cap stricken by the hammer that produced sparks which were led into the barrel by the opening the cap was attached to.