That's an adjustable capacitor used for fine-tuning the filter for the crystal. The crystal is a very cheap piece of quartz, not running a stable frequency, but rather a whole bunch of different frequencies. The one you need for the clock is then filtered out - because every crystal runs its own frequencies, the filter needs to be adjustable. Due to this low-cost method the clock's usually not very acurate (temperature changes, ...).
Turn right for making the clock run faster, left for slower. A very careful turn will make up for 5-20 seconds a day.
