A quid is one pound sterling, although the term is sometimes used to refer to other currencies. Historically it also referred to chewing tobacco. Eugh.
To make a quick quid is to earn some money in an easy manner, either honestly or otherwise. eg When Fred saw what John was up to he realised he could make a quick quid by promising to keep quiet.
To be quids in is to be in a position to profit from something, usually in a finacial manner, but it can refer to other benefits. eg Being the only taxi driver who owned snow chains meant Cheryl was quids in during the bad weather.
Anyone described as not the full quid isn't considered very bright.
Thanks to Alyson (whose support of this blog is beyond price) for passing on the details of this flash fiction competition. It's free to enter and the prizes are membership of The Writers' HQ and places on one day writing retreats (with alternatives if you can't attend). Reading the competition details I was startled when I saw the first mention of money as it was in quids, not dollars. Until then I was sure I was reading about a U.S. organisation.
I can remember when a quid could be folded and was considered proper money, not loose change – can you?