Lock time is the time interval between the release of the hammer (by the sear) or striker and the time it hits the firing pin which in turn will hit the primer of the cartridge hard enough to detonate it.

Probably the term came from an era when flintlocks were still the in use.

Basically, the shorter the lock time the better the accuracy you can expect from the firearm as there is less possibility of movement from the point of aim before the bullet leaves the barrel.

Lock time differences are generally not discernible but the effects of it is.

To get better lock time one can use a heavier mainspring, lighter firing pin, lighter striker, or a lighter hammer. But for most of us, our accuracy would be better improved by spending the money on ammo and practice time at the firing range.

Remington also (ED: used to) offer the Etron X rifle system which uses an electronic ignition system and is advertised to have as close to zero a lock time as physically possible.