MLS is an acronym used in real estate and stands for Multiple Listings Service. The MLS ID number is used to reference a particular piece of real estate, whether it's a home, land, or business listing. You can find this number on almost all real estate listings. You can use the number to find out additional information about the piece of property. There are a variety of online resources to obtain this information. You can also bring the number to a local real estate agent and they can research it for you.