They are prepared by a certified kosher kitchen... the meal is sealed and wrapped. Most airlines I know of ask you to show it to the passenger so they can see that it is sealed and certified, once they have seen it you may then remove any components which need heating.
I'm not entirely sure but of course pork products are definitely not allowed, also I think shellfish and any kind of fish without scales and any meat from animals with split hooves (I think it's split hooves anyway!!!)
I looked at one closely once, it had a piece of paper in it stating which kitchen it was made in and that it had been inspected and approved (I think a rabbi does this) also the milk had special labels on it as did most of the snack components. The whole tray is encased in sealed plastic, not cling wrap.
Just some that I have dealt with, I would guess would be similar elsewhere. Your airline should have a procedure somewhere for dealing with this, I would think it would be similar to what I described.