Anyone can start earning trading fees on exchange platforms like Uniswap today by being a liquidity provider. Pools fully distribute trading fees to liquidity providers. As a liquidity provider, you add a specific ratio of assets to help faciliate trades in the pool. Doing so gives you an ownership share of the pool and the future trading fees it generates. You can pack up and leave anytime. Your ownership shares are completely liquid. Trade them in for your share of the underlying assets in the pool.