If the pistol is cocked and locked and pressure applied to the trigger with the grip safety depressed, the hammer should not move; that is, the sear should not re-position itself in the hammer notch closer to the edge. If it does, slight pressure on the hammer will cause a very faint "click" that signals movement and an unsafe lockup that will eventually loosen further and cause unintentional discharges. The thumb safety should be replaced with one properly fitted. Sometimes, a thin coating of silver solder on the filed contacting surface will rejuvinate the safety without having to replace it, and I've seen this done many times, but it is considered questionable, and ultimately...even if it works temporarily...the thumb safety should be replaced.
If you are talking about the hammer dropping to half cock, you can search for the thread that covers this condition exhaustively.