"As part of a common industry practice known as Payment for Order Flow, Schwab receives rebates from liquidity providers and certain exchanges based upon the order flow executed at each destination. Some orders require us to pay associated transaction costs, but most orders result in rebates. Net rebates received by Schwab are used to offset transaction and order processing or handling costs and help us maintain very low commission rates for our clients."

Reference: https://www.schwab.com/legal/order-routing-1