I don't understand it either honestly. But according to a quick google, it mostly boils down to this: banks are run for profit (to shareholders, who then pass along some of that as interest), whereas credit unions are non profits that return all their "profit" to members (which I guess technically no longer makes it profit, but I'm not sure what else to call it)

If your country is Australia, we do indeed have credit unions, People's Choice Credit Union is the one that comes to mind, but I know there are others. I think they're probably like a super minority - I've never met somebody in real life that has banked with a credit union. But some of them are kind of like industry super funds in that you need to be employed in a specific industry to be eligible for an account with them (see: Teachers Mutual Credit Union)

With a bank, they operate to maximise profit for their shareholders (not members / customers).

With a credit union, you buy a share when you open an account which means every member/ customer is also a shareholder. As they run to maximise profit for their own customers, that means decisions and pricing is made that benifits the customer.

The result is cheaper (or no) fees, better interest rates on loans, higher interest rates on savings accounts, and usually way more flexible on things like paying back extra on home loans.

In Australia, Credit Unions (and building societies) are covered by the same government guarantees that Banks are so the old fears of your money not being safe are stupid.