The earliest and most primitve of the firing mechanisms (or "lock" types) to develop in firearms design.
The cannon lock is simply a hole in the breech end of the weapon into which flame can be introduced to ignite the propellant - the user must supply his own source of flame. Frequently some kind of carrier - usually a finer grade of black powder - was used to fill the hole between the outside of the gun and the propellant (the touchhole).
Cannon lock became obsolete in hand held weapons very quickly as it effectively required the user to have three hands - two to hold the weapon and one to apply the igniter - not to mention carrying around a source of naked flame whilst he (re)loaded his weapon with powder and shot. The three-hands problem could be reduced by the use of a monopod, but remained a major drawback of the system. The immediate sucessor to cannon lock in small arms was matchlock, which more or less abolished the need for a third hand, but kept the burning match.
As the name suggests the technology lasted rather longer in ordnance class weapons (like cannon) - which didn't need to be held and which were generally accompanied by enough extra kit that a firepot or tub of burning slow match wasn't an issue.
Even when the Royal Navy installed flintlock triggers as standard on its cannons, normal procedure was to issue tubs of slow match to all crews as a precaution against failure of the trigger mechanisms.