Okay, so have you heard of something called a "piggy bank"? It's a place where you can save your money and maybe even earn some more by putting it in a bank account.
Well, an OEIC is like a big piggy bank, but it's for adults who want to invest their money. It stands for "Open-Ended Investment Company," which basically means that lots of people can put their money in and buy a little piece of the company.
Just like how you can put different things in your piggy bank, like coins or bills, people can put different types of investments into an OEIC. This could be things like stocks (which are like little parts of a company), bonds (which are like loans to a company or government), or even property (like buying a house to rent out).
When you put your money into an OEIC, it gets mixed up with money from other people who also invested. Then, the people in charge of the OEIC (called "fund managers") use all that money to buy different types of investments.
The really cool thing about an OEIC is that you can buy and sell pieces of it whenever you want. So, if you need your money back or want to invest more, you can do that pretty easily.
But just like any type of investing or saving, there's always a risk that you won't make as much money as you hoped, or even lose some. That's why it's important to do your research and talk to grown-ups you trust before investing in an OEIC or anything else.