Explain what happens when Bran is climbing near the First Keep and overhears a conversation.

Bran is climbing a broken tower near the First Keep where he overhears Cersei and Jaime talking. When Bran looks in the room, he sees Jaime and Cersei in bed together. Bran slips, and while hanging onto the window, Jaime and Cersei find him. Jaime pushes Bran out of the window.