Users that add liquidity earn a percentage of each trade, proportionate to their ownership of the liquidity pool. This fee varies and can be seen in the buy/sell widget. For example, Alice owns $1,000 of liquidity in a $10,000 pool with a 2% LP fee. Bob buys $5,000 worth of tokens, which means Alice would earn $10 (5,000 * 0.02 * (1,000/10,000)).