The multiple listing service is a shared database of houses for sale that real estate agents use to list properties and search for them on behalf of buyers. Getting a house on the MLS is what puts it in front of most agents and, through sites that pull MLS data, most buyers searching online. A house sold directly to a cash buyer typically never touches the MLS.
Why it matters
Skipping the MLS trades a wider pool of buyers for a faster, more private sale.