They're a market market that provides liquidity to clients. They (as far as I know) don't charge fees, but earn their profits by exploiting the ask/bid spread.

A customer might want to offload TSLA and are willing to pay the market rate of $329. This trade might work in HRT's favor if they can sell it for $329.01. It's just 1 cent, but over millions of transactions these small amounts of profit add up.

The value captured by HRT is meaningful in the aggregate, but tiny and irrelevant to the institutional clients, and therefore can't be thought of as a fee. What HRT provides in return for taking clients' trades is liquidity.