Okay, so let's imagine you and your friend both make and sell lemonade. Your lemonade tastes good, but your friend's lemonade tastes even better. That means your friend has a unique selling proposition, which is something special that makes their lemonade different and better than yours.
Having a unique selling proposition is like having a secret ingredient that makes your product different from everyone else's. It's what makes people want to buy your product instead of someone else's. So if you were selling lemonade, maybe your unique selling proposition would be that you use organic lemons, or you add a special kind of sugar. Whatever it is, it needs to make your lemonade stand out and be better than all the other lemonade out there.
Companies use unique selling propositions all the time to sell their products. For example, a company that makes cars might advertise that their cars are the most reliable, or eco-friendly, or have the best safety features. That's their unique selling proposition, and they hope it will convince people to choose their cars over another company's cars.