A universal bank is like a superbank that can do all sorts of things: they can help people save their money, lend money to people who need it, and also buy and sell stuff like stocks and bonds.
Picture it like this: imagine your piggy bank at home is a bank that only lets you save your money. You can put money in and take it out, but that's all it can do. Now, imagine a giant piggy bank that can not only save your money, but also lend money to your friends if they need it and buy special things like rare coins or stamps. This is what a universal bank can do – it can offer many different kinds of banking services all in one place.
Just like how you might go to a mall with lots of stores instead of going to different stores, people might choose to go to a universal bank so they can take care of all their banking needs in one place.