Imagine you and your friends really love playing with a specific type of toy, like Legos or Barbies. You all really enjoy spending time with these toys and want to keep playing with them as much as possible. So you start a group, or a club, just for people who love playing with that toy.
Now let's say you all decide that there's something you really want, but it's too expensive for any one of you to buy on your own. You can't just ask your parents to buy it for you, because they may not want to spend the money. So you decide to pool your money together to buy it as a group.
This is kind of like what an interest group does, but instead of toys, they have a shared interest in a specific issue, like the environment or healthcare. They come together to try and to get something they want, like a new park or a better healthcare policy, and they pool their resources to achieve their goal.
Interest groups try to influence government officials - like your parents - to take actions that are favorable to their cause. They do this through things like lobbying, which means talking directly to politicians and trying to persuade them to vote a certain way or take a certain action. They might also organize protests or rallies, or publicly endorse certain candidates for office.
So just like you and your friends work together to achieve something you all want, interest groups do the same thing on a bigger scale to try and make a difference in the world.