A primary dealer is like a special buddy that the government has who helps them out with selling and buying things. They are really good at helping the government sell and buy things like bonds, which are a special kind of paper that people can buy and keep as an investment.
Think of it like a lemonade stand. When you want to sell lemonade, you need someone to help you make and sell it, like your mommy or daddy. In the same way, the government needs help from a primary dealer to sell bonds to people who want to buy them. Primary dealers are really good helpers for the government when they need to buy or sell things!