Liquidity providers are decentralized exchange users who fund a liquidity pool with tokens they own.
A liquidity provider is a user who funds a liquidity pool with crypto assets she owns to facilitate trading on the platform and earn passive income on her deposit.
Liquidity pools are leveraged by the decentralized exchanges that use automated market maker-based systems to allow trading of illiquid trading pairs with limited slippage. Instead of using traditional order book-based trading systems, such exchanges use funds that are held for every asset in every trading pair to allow trades to be executed.
While trading illiquid trading pairs on order book-based exchanges could lead to suffering from great slippage and the inability to execute trades, the advantage of liquidity providers is that trades can always be executed as long as the liquidity pools are big enough. For this reason, liquidity providers are seen as trade facilitators and paid with the transaction fees paid for the trades that they enabled.
How much liquidity providers are paid is based on the percentage of the liquidity pool that they provide. When funding the pool, they are usually required to fund two different assets to enable traders to switch between one to the other by trading them in pairs.
For instance, a liquidity provider may provide a liquidity pool with $5,000 worth of Ether and $5,000 of USD-pegged decentralized stablecoin DAI to allow trading back and forth between the two. This way, every time a trade on the ETH/DAI is executed, the liquidity provider in question would receive compensation for having funded the pool in question.