The question was general, so let's try a general answer. In a modern revolver at rest, the hammer is down, and the cylinder is motionless, locked (prevented from turning) by a cylinder stop (also called the bolt, or latch by some makers) fitting into a notch in the cylinder. When the gun is fired in double action, the trigger is pulled, the stop is pulled out of engagement from the cylinder notch, a pawl rotates the cylinder by engaging a ratchet, the hammer comes back, the cylinder continues to rotate while the cylinder stop is released to contact the cylinder. As the cylinder continues to rotate, the hand stops trying to turn it just as the cylinder stop engages the next notch in the cylinder. This must happen before the trigger moves far enough that the hammer will be released to fire the gun.
As you see, everything must happen in a precise series of actions, all at the right time. This process is called timing and when it is correct, the revolver is "in time". As you can also see, there are many places in the series where something can go wrong; there might be more than one such place. When that happens, the revolver is "out of time". That can mean failure to fire, firing with the cylinder unlocked, or something else that can cause damage to the gun.
When a gun is "out of time", it can be fixed by replacing parts, or working on the old parts. Sometimes, on old guns, parts are not available and the wear is such that the gun cannot be fixed by any economically feasible means. Then, the gun must be discarded or retired to wall hanger status.
Jim