The MLS stands for "Multiple Listing Service". Each area of the country has an association of Realtors. Most, but not all, Realtors belong to their local MLS. They pay dues to belong to the association and the association provides tools like a multiple listing service. Realtors place their listings (homes for sale) in the MLS and advertise to other Realtors the details of each listing (such as number of bedrooms, square footage, and whether the home has a pool, as examples). Most MLS's feed this data into search engines such as Trulia, Yahoo, and Front Door (to name a few) where anyone can search for homes based on those specifics.
If you work with a Realtor that is a member in good standing with his / her local MLS the Realtor can look up the pricing history, days on markets, and other aspects of any property that has been advertised to sell in the MLS. Not all properties are listed for sale in the MLS (such as For Sale By Owners) so there may be some properties that your Realtor can not find out about.
If you want the best service in your search for a new home, I recommend interviewing a few Realtors and finding one that you feel most comfortable with and then committing to work with him or her. It will make your home search much smoother and more efficient.
Good luck in your search!
Prudential Network Realty
Orange Park, FL
C: (904) 327-5676
O: (904) 269-1716