i.e. when the "tick" and "tock" of a watch are of different timing lengths, they need to be adjusted so they are equal duration. This can be fixed by adjusting the position of the start of the hairspring (the stud).
A watch can be at the correct beat rate (i.e. number of beats per hour), but can still have a beat error. It matters because if your watch is in different position as you wear it, the timing may start to have variations.
Regards,
skyeriding