means "Pretty big deal."
It can be used on its own, but it is typically used as a response to counteract "No big deal" or nbd
Also for emphasis: pbfd = "Pretty big fucking deal."
1. Person A: "I just saw Joseph Gordon-Levitt walking out of Starbucks, nbd."
Person B: "Uhh... pbfd."
2. In the Christian religion, the birth of Jesus was apparently a pbd.