A maker is someone who places limit orders. If you place an order for a price some distance away from the current market price, your order goes on the order book and waits to be filled. When you place an order onto the order book, you are called a maker because you are making a new trading opportunity on the exchange’s order book. You are also said to be adding liquidity to the market. Without a limit order by a maker on the book the price will swing heavily. Therefore, market makers are rewarded with low fees for adding liquidity to the market.