Mark
Haven't seen the ad, but Performance usually refers to the department that produces airfield take off and landing analysis. These data enable the pilot to calculate his maximum take off weight/landing weights for the conditions on a given day from a selected runway. They also often work on flight planning, navigation and related systems and support.
If its a trainee position then you'd usually be expected to numerate, computer literate and have an eye for detail and be capable of very accurate work.
Good luck.