Broker

A broker is a licensed firm or individual that executes buy and sell orders on behalf of clients, acting as the intermediary between an investor and the exchange or market maker on the other side of the trade. Every retail trade in the US passes through a broker, whether the client ever thinks about that layer or not.

Modern retail brokers mostly earn revenue through payment for order flow and interest on cash balances rather than per-trade commissions, which is how commission-free trading became the industry standard. The broker's execution quality (how good a price it actually gets you) still varies even when the sticker price is zero.