It's a very large shed that is usually home to guns, trucks, tanks, aircraft and many rare oddities in between, many being restored to go on display and others just being stored due to a lack or room at the AWM. I think the V1 rocket is still there which is mighty impressive.
As an example I saw the Me262 and 163 there well over 10 years ago, many years before they finally went on display at the AWM. I couldn't say for sure what exact items are there now as it changes regularly, but there's guaranteed to be something cool you havn't seen before.