In a word... everything!
They can ask you about every aspect of the RN, RFA, RM... be it subs, ships, aircraft, weapons.
They will ask where certain ships are in the world and why they are there, so you need to look up on current affairs, past, present and possible future conflicts.. i was asked what the next conflict in the world would be... and then had to back my answer up!
Just make sure your naval knowledge is up to scratch... be able to tell the difference between a frigate and destroyer... keep an eye on the whereabouts of all ships in the world and also all current and future operations!
I bought a book and revision guide from How2Become. Have a look on the website, for the sake of a few ££'s its worth it