The Limbo of the Fathers is where the righteous awaited the resurrection of Jesus, after which they could enter heaven.
The concept derives itself from the Jewish belief in Sheol.
Judaism had a development in its understanding of the afterlife. Originally, Sheol was believed to be a place where all the dead went. In later periods Judaism developed the belief that in Sheol the righteous and the wicked were separated. This is clearly seen in Jesus’ parable about the Rich Man and Lazarus (Luke 16:19-31). In early Jewish belief it was thought possible to pass from the side of the wicked to the side of the righteous (the “Bosom of Abraham”) through the intercession of Abraham. However, by the time of Jesus, Judaism generally believed that it was not possible to pass from one side of Sheol to the other (Luke 16:26).
Scripture refers to Jesus descending to the realm of the dead and preaching to the dead (Eph. 4:9; 1 Pet. 3:18-20). What did Jesus preach to them? It would make little sense to preach to the damned, so tradition tells us that Jesus preached to those who were to enter heaven.
Tradition teaches us though the gates of heaven were closed, in the limbo of the fathers (“the bosom of Abraham”) the righteous awaited the resurrection of Jesus and entrance into heaven.