They want to know as soon as possible about any time that you will definitely need off work for holidays.
So they are asking you to tell them about any holidays that you have already booked and committed to. Something like a family wedding would also be considered a commitment.
During what holidays, Maisy?
In England, the employer is asking what holidays the new employee has already committed to - perhaps if I expanded to say - what vacations the new employee has already booked...?