You could think of an automated market maker as a robot that’s always willing to quote you a price between two assets. Some use a simple formula like Uniswap, while Curve, Balancer and others use more complicated ones.
Not only can you trade trustlessly using an AMM, but you can also become the house by providing liquidity to a liquidity pool. This allows essentially anyone to become a market maker on an exchange and earn fees for providing liquidity.