Actually, to be fair, they are optional. Most of the major airlines will have an application form which will need to be completed prior to any interview, and this will ask for all the formal details they need including referees. However, most Wannabes are likely to be targetting smaller companies, which may not have these forms, and the mention of referees may be useful.
It would be quite sufficient, probably, to just mention that referees are 'available'. These referees should be people that know you in a professional sense. If you have no work history, flying school or further education staff may agree to write references for you.