"MLS" stands for Multiple Listing Services but for the average user it doesn't explain what these domain names will be used for and who will have the use of them.
I think the explanation can be read here in this sentence : "Because consumers have come to understand that those three letters represent the gold standard for accurate, up-to-date property information".
- UP-TO-DATE PROPERTY INFORMATION -
Note .mls domain names will be used mainly in the United States of America.
http://www.mlsdomains.org/take-back-our-mls/