First-mover advantage means that when a company is the first one to make and sell a product or service, they have an advantage over their competitors. Imagine you and your friend want to sell lemonade. If you start selling it first, you will have all the customers because people will come to you first. Your friend will have no customers if they start selling after you because they'll be too late.
This advantage happens because people are curious about new things, so they'll try it out when it's first available. And if they like it, they'll keep buying from the first company, who has already built a reputation and trust.
However, being the first mover also has some risks. It can be expensive to develop and market a new product, and mistakes can happen while experimenting with the product. If the product is not successful, the company may lose a lot of money.
So, having a first-mover advantage is like being the first kid to share their toy with the others. You get a head start, but you also have to be careful with your choices.