The whole deal is basically this:
MAJOR SPOILERS FOR GAME OF THRONES SEASON 3 EPISODE 9 AND A STORM OF SWORDS
Robb and his allies arrive at the Twins for Edmure Tully’s wedding to Roslyn Frey. After the ceremony and bedding of the bride and groom, the doors to the feast hall are shut. Frey soldiers and sellswords that were diguised as the musicians for the affair draw crossbows and knives and brutally (and I mean brutally) murder Robb’s wife and mother. Meanwhile, all of Robb’s bannermen camped out in their tents are caught unsuspecting when the Frey’s light oil that they coated the field with before hand. Robb Stark himself is personally killed by Roose Bolton. The long and short of it is that Robb Stark, his wife, his mother, and the vast majority of his 3500 bannermen are massacred, effectively ending the war. This act is particularly heinous because it violates the most basic, sacred law of all civilizations in the GoT world; Guest Right (you can’t kill guests that you have formally invited to stay, and a guest can’t do the same to their host).