Brief history of it really. Learn when they were founded and all the dates and everything to do with the crest. If you have an iPhone download the royal marines app on the app store.
In my interview he asked me about the crest and I started going on with exact dates and descriptions of the parts of the crest an he stopped me and said (as good as) "you have clearly done you homework and have an interest in the marines, that's all I need to know". You just need to show that you have an interest and you are not some lad who just walked in the AFCO for no reason.
Seriously, download that app if you can. I researched - dates and descriptions for all parts of the crest, founding date and current deployments, basic deployments from founding to current date (ie. What did they do in the 1800's, during the world wars, foundation of the commandos and the post-worldwar deployments like Falklands etc)
There's a list of questions ninja posted in the stickies/FAQ's section on here mate. All I did was learnt them all off by heart. He asked me a handful of them and I just fired them off back at him. Battle honours/vc's won and other little bonus bits will take some research.