These phrases appear in a dialogue between two friends. What is

These phrases appear in a dialogue between two friends. What is the dialogue about? , Listen and read to find out.
Katie: Good morning, Lizzie.
Lizzie: Is it?
Katie: I think someone got out of the wrong side of the bed this morning! Whats the matter?
Lizzie: Oh, Im just a bit tired, thats all. I had a horrible nightmare last night. I woke up shaking like a leaf and with my heart pounding. Then I couldnt get back to sleep.
Katie: Poor you! What was the nightmare about?
Lizzie: Well, I was trying to get to an exam, but my legs just wouldnt move.
Katie: So... did you finally get there?
Lizzie: Yes, but then I realised that I couldnt answer any of the questions!
Katie: Oh, thats terrible! I once dreamt that, either. I looked it up. It could mean that you are afraid of letting someone down.
Lizzie: Thats a thought! So it doesnt mean Im going to fail my exams then?
Katie: No, dont worry. Nightmares are just your subconscious mind trying to deal with all your stresses and worries, thats all.
Lizzie: Thats a relief!