Second son to Vito Corleone (aka Don Corleone) of "The Godfather". Normally, the title of Godfather would have been passed down to him since Santino Corleone was killed. However, Vito felt that Fredo was a little on the dumbass side so the title fell to Michael Corleone.
Fredo didn't take too kindly to this and in "The Godfather Part II", he betrayed Michael which resulted in a murder attempt. Michael discovered the betrayal and the famous scene of the Kiss of Death was enacted. However, Michael's mother begged him to not do anything to Fredo.
Michael agrees. Mom dies and Fredo begs Michael for forgiveness. It looks like things are cool until the end when Fredo is capped in the head while on a fishing trip.