As I understand it it's basically a check of your whereabouts over the past ten years. Specifically they check that you've been working/job hunting/studying in the UK for the past 10 years. I think the aim is to verify that you haven't spent a significant amount of time out of the UK. The way I understand it is that they are effectively checking to see if there is any possibility that you've spent any time at terrorist training camps etc.
As for what they check, I actually have no idea. I would imagine that it's a check to say that you actually worked where you say you worked, and how much time you worked there.
Perhaps someone from an airline recruitment background can provide more information.