Literally that. Something that you can wear under a smock as an extra layer if the weather is cold. It doesnt have to be anything special, but something black/brown/green/tan would be ideal - anything is better than nothing, though.
DPM is the old camouflage pattern, so they are asking for waterproof trousers and jacket - likely either DPM or MTP (or even a mix is actually allowed).
If your squadron doesn’t have any to issue, if you have normal “civvie” waterprof trousers and a light rain jacket (ideally dark though as opposed to pink or blue or yellow or orange or some other high vis colour) then that will have to do. Anyone who conplains should be told to give their head a wobble.