You're sat there listening to some intentionally poorly recorded song and watching people dance.
In a nutshell, that's correct. In fact, the vocals were intentionally badly recorded, in order to fool the audience. This is a twisted/crazy situation, created due to the fact that Cheryl does not trust her own voice, in a live performance.
LOL
I would've loved to have heard the conversation that took place when they decided not to sing live:
Cowell: So Cheryl, do you want to sing live.
Cheryl: I'm not sure. I don't think I can sing and dance at the same time.
Cowell: Why don't you just sing and let the dancers around you do the dancing.
Cheryl: Erm, I don't think I can sing properly, even when standing still.
Cowell: OK. In that case you will have to mime.
Cheryl: But what will the audience think of me? I can't sit here, judging people for their live performances if I can't sing live myself?
Cowell: OK. How about we pre-record your vocals, poorly, so that on the day of your performance, we can fool the audience into believing that the performance is really live, as shown by the poor vocals.
Cheryl: That could work!