"Cocked and locked" refers to a method of carrying a pistol, often a 1911 gov't model semiauto, with a round already in the chamber and the
hammer cocked back and ready to fire. It is a term coined by
police who carry 1911 pistols in this manner. Usually the holster strap is inserted between the
hammer and firing pin for safety, which is snapped open when needed for a fast draw. It is actually safe to carry a 1911 model pistol in this way because of the redundant safety mechanisms such as the 1911's grip safety and thumb safety to protect against any accidental discharge until the weapon is on
target and the shooter is ready to fire.
I like to
carry my 1911
Colt Commander "cocked and locked" in my concealment holster when I go out. I want to be instantly ready for action if I have to draw. Having to rack your
slide before firing could potentially get you killed.