Mother With Benefits. Similar to MILF or Cougar. An older, motherly figure who can provide a partner (typically a young male) with the benefits of comfort, care, kindness and SEX.
Guy 1: Dude, I'm really missing home right now
Guy 2: Why?
Guy 1: Well, I miss my Mom's cooking and hugs and my girlfriend
Guy 2: Let's go to the Grocery Store and find you an MWB!
Guy 1: What's that?
Guy 2: A Mother with Benefits!!