To add a bit of context: brokerages like Robinhood send buy/sell orders to national exchanges and to private trading firms e.g. high-frequency traders. Private firms provide price improvement: orders that execute at prices better than the national exchange. All brokerages have a duty of best execution, including a duty of price improvement. Brokerages can also receive payment for order flow from private firms, as long as it does not interfere with best execution. However, "Robinhood explicitly offered to accept less price improvement for its customers... in exchange for receiving a higher payment for order flow," which is illegal.
Full order: https://www.sec.gov/litigation/admin/2020/33-10906.pdf