If you could get together with a group of friends and raise a pot of money, you could then lend it to any person who needed cash, say to buy a home or to purchase a new car. Assuming the arrangement was non profitmaking, it would mean members of the 'credit union' would benefit from cheap loans.
As a mutual society the credit union is a financial co-operative, just like the original building societies and friendly societies. It is owned and controlled by its members and run for their benefit.
These days building and friendly societies are hardly ever chummy, local affairs and members don't necessarily have anything in common. Not so with a credit union where every member will tend to have a 'common bond' with other members of the union. Perhaps everyone lives in the same village or everyone works for the local factory.
There are currently over 600 credit unions with the average having 400 members. Leeds City Credit Union, on the other hand has managed to attract 8,500.