Advance rent is an upfront payment towards your first rent instalment. It's usually £250 and is paid when you book, so it isn't an additional fee on top of your rent.
Advance rent has replaced deposits for most bookings, so you usually won't need to pay a deposit as well. However, some students who book through their university may still be asked to pay a deposit.
If you're paying your rent in full or booking after your first rent instalment is due, you won't need to pay advance rent.