The rotor is the moving weight inside an automatic watch. As your wrist moves, the rotor swings. The winding system converts that motion into energy stored in the mainspring.
What the rotor looks like
Many rotors are shaped like a semicircle and pivot around the center of the movement. In a watch with a display case back, it is often the large part that moves when you tilt the case.
Different movements use different rotor shapes and winding systems. Some wind in one direction, while others use movement in both directions.
Rotor vs mainspring
The rotor does not power the hands directly. It helps wind the mainspring. The mainspring stores the energy, the gear train carries it, and the escapement regulates its release.
Do all mechanical watches have one?
No. Manual-wind mechanical watches are wound through the crown and normally do not use an automatic rotor. Automatic watches use a rotor or another moving winding mass, and many also allow manual winding through the crown.
Is rotor noise normal?
Some movement noise can be normal. Rotor design, case construction, and winding direction all affect what you hear and feel. A sudden new scraping sound, grinding sensation, loose rattle, or poor winding performance is different and should be inspected.
Does more wrist movement always help?
Only up to a point. Ordinary movement helps maintain the wind, but a low-activity day may not provide enough motion to fully wind the mainspring. If the movement allows hand-winding, a gentle manual wind can help before wearing it.
Next, read what the mainspring does.