There was a prevailing Passover custom in Jerusalem that allowed or required Pilate, the governor of Judaea, to commute one prisoner's death sentence by popular acclaim. Rather like our president commuting the turkey's fate at Thanksgiving I suppose.
The choice was between Jesus of Nazareth, whose history you're aware of; and Barabbus, a notorious prisoner guilty of rebellion, insurrection and thievery. The popular choice was Barabbus. Somehow the choice doesn't seem to surprise us, but it's possible that the story is apocryphal.
BARABBAS: (And this is what's kind of interesting)