It is the French equivalent of LOL ('laugh out loud' in anglophone internet parlance). Initially, MDR or Mdr was a very common acronym used in text messages, online forums and gaming.

As it turns out, the people in charge of the Académie, known humbly as “Les immortels” (The Immortals) are not exactly fond either of the French version of LOL: “MDR,” or “mort de rire ...

But French also has a more delightful acronym: The French equivalent of LOL is MDR, which means "mort de rire," or "dying of laughter." Hat tip and 55555 to Chris Heller.