MLS stands for the Multiple Listing Service. Real estate agents and brokers pay for and maintain their local MLS in order to help other agents and brokers find for-sale and for-purchase homes for their clients.
There are over 500 different MLS providers in the United States today. Agents typically need to pay a membership fee to their local MLS in order to gain access to the listings. They do so in order to gain access to deals and listings for their clients.
Property investors can work with brokers to gain access to the MLS. The broker and agent wants to sell the home so they're often interested in sharing deals that fit the buyer's criteria.