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Solar panels must be always exposed in the direction of sun to produce maximum power.
Since the ISS orbits the Earth, if the panels are fixed they will not produce maximum power (often fluctuating).
How does the ISS control its solar panels' direction ?
There are two major joints that allow the solar arrays to rotate, the Alpha and Beta gimbals.
Note this is a discussion of the main 4 arrays at the opposing ends of the main truss. The Russian segment has its own solar arrays, and there used to be a plan for more. There have been a series of proposed Solar Power modules for the Russian segment that keep getting cancelled for budgetary reasons. The current plan calls for the Nakua modules to dock to where Pirs is (earth facing port of Zvezda) and then have the UM (Universal Module) with 6 docking ports, dock to the bottom, and then have the Solar Power module dock to the UM.
These two joints allow the panels to rotate to track the sun as needed. The alpha gimbal allows them to rotate as if in a circle, centered on the truss itself. I.e. Imagine the panels windmilling with the center on the truss.
The Beta gimbals allow the panels to tilt around to accommodate inclination changes in the sun.
Also on the truss, are the thermal radiators to dump waste heat that rotate to AVOID direct sun exposure so they can better dump heat into space.
There are electric motors that are used to adjust the solar panels, but not necessarily to face the Sun. The solar panels are a major source of atmospheric drag for the station, as even at 200-300 KM above the Earth's surface there are enough air (nitrogen, mostly) molecules to cause slight drag at the speeds that the ISS passes. Every few months the ISS needs to be reboosted due to this effect. Therefore, the solar panels are turned with electric motors to reduce this drag.
Reviewing the excellent ISS Wikipedia article I see it mentioned that the solar arrays are only turned to reduce drag while on the night side of Earth, and on the day side they are turned toward the Sun. Other sources that I've seen mention that the solar arrays collect light from both sides and that the Earth reflects significant light so that facing the panels exactly towards the Sun is not a top priority. However no matter how much priority is put on where to face the panels, for aerodynamic or for power reasons, the panels are rotated with an electric motor.
