A finance broker, or a dealer, is an entity that serves as an intermediary in negotiations or transactions.
Usually, a broker is a person or a firm who buys and sells securities to customers, often for a fee or commission.
A broker is essentially a salesman, and can be perceived similarly to the way a store acts as a middleman between shoppers and the companies which produce the goods the store sells.
In the same respect, a broker acts as a middleman between the securities that trade on the market, and the investors who purchase these securities.