Timing means that the hand moved the cylinder into the exact position to latch up aligned with the barrel and the stop retracts and extends the proper amount at the proper times. The final step in timing is to ensure the stop actually locks the cylinder into alignment with the barrel.
Stop retracts and releases the cylinder, dropping completely out of the cylinder notch.
Hand engages the cylinder rotation notch and pushes up, rotating the cylinder into the next position. No more and no less (within a few thousandths.)
Stop extends back up into the cylinder locking notch and locks the cylinder into alignment with the barrel.
The cylinder locking notches are not always cut square on the "leading" edge, so the stop can start coming up before the hand reaches the limit of rotation. On many revolvers, the stop comes into contact with the cylinder early and scratches a tiny groove around the cylinder. This is a sign of improper timing. It is not terribly important to the proper operation, usually. However, it is one of the fussy-fussy things one can fuss-fuss with, if such interests you.