Payment for Order Flow
As a way to attract orders from brokers, some exchanges or market-makers will pay your broker's firm for routing your order to them perhaps a penny or more per share. This is called "payment for order flow." Payment for order flow is one of the ways your broker's firm can make money from executing your trade. The firm can also make money by internalizing your order.
Upon opening a new account and on an annual basis, firms must inform their customers in writing whether they receive payment for order flow and, if they do, a detailed description of the type of the payments. Firms must also disclose on trade confirmations whether they receive payment for order flow and that customers can make a written request to find out the source and type of the payment as to that particular transaction.
To learn more about the basics of trade execution including order routing, payment for order flow, and internalization you should read Trade Execution: What Every Investor Should Know.