A shareholder is like when you and your friends decide to build a big LEGO castle together. Everyone brings some of their LEGO bricks to help build it. The more bricks you bring, the more you get to say about how the castle looks. A shareholder is someone who brings their bricks to help build a company, and in return, they get to help decide what the company does.
Another way to think about a shareholder is like owning a piece of a giant pizza. If a company is a pizza, each slice is a part of the company. When you own a slice, you are a shareholder. If the pizza gets bigger and more delicious, your slice becomes bigger and tastier too.
Being a shareholder can also be like being part of a sports team. You get a jersey with your name on it, and you cheer for the team to win. When the team wins, everyone on the team gets to celebrate and share the prize. In a company, when things go well, the shareholders get to share the success.