OPEC stands for the Organization of the Petroleum Exporting Countries. It's a group of countries that work together to control how much oil they produce and how much they sell it for.
Imagine you and your friends have a lot of toys to sell. You could sell them all for a low price, but that wouldn't make you much money. Or you could sell them for a higher price, but maybe no one would want to buy them. So you and your friends decide to work together and agree on one price that's fair and will make you all a good amount of money.
That's kind of what OPEC does, but with oil instead of toys. They decide how much oil they'll produce and sell it at a price they all agree on. This helps them make sure they're making enough money for their countries and that they're not selling too much oil and running out of it.
Sometimes other countries that aren't in OPEC don't like this idea, because it means they might have to pay more money for the oil they need. But for the countries in OPEC, it's important to work together to make sure they're doing what's best for their own countries.