A small bubble level to help put a mechanical clock into beat. You can also just level it for an even tick tock. Sometimes a clock will show that it is level. However the tick and the tock are not even. If this is the case, please read 'how to put a clock in beat'. This is in the help sections.
No clock will run if it is not in beat. However, this does not have anything to do with the clock being level. Although that is always optimal when putting the clock in beat. That way the next time it moves from here to there it will still be in beat. This is providing that they also move it to another level place.