The red cliffs is a series of tall, pointed rocks in the Pride Lands. They are a dull brownish-red, hence their name.
When Nala finds a secret cave near the red cliffs, she makes Simba promise not to tell anyone. After he makes the promise, she informs him of the cave's location, but she forbids him to follow her there. Simba does as he's told, but come nightfall, Nala is nowhere to be found.
Sarabi, Simba's mother, comes to Simba and asks him where Nala is. Simba admits that he can't tell her, for it's a secret, but Sarabi gently reminds him that secrets sometimes need to be told for the safety of others. Seeing the wisdom in his mother's words, Simba tells the pride where Nala is, and they set out to rescue her.
Once at the red cliffs, the lionesses find Nala trapped in a rock slide. Thinking quickly, the pride digs her free, and Nala rushes to her mother, promising to never go off on her own without telling someone again. Once the chaos is over, Simba apologizes for telling Nala's secret, but the little cub reminds him that she would still be trapped if not for him. Together, the cubs return home and begin to dream about their next adventure together.