What is high altitude testing? When new engines are designed for aircraft (military or civilian) they undergo high altitude testing (especially the military ones).

What is high altitude testing of engines? How is that important?
High altitude means low air density. Ignition is harder to achieve with lower density, and the size of the combustor determines up to which altitude and throttle setting an engine will stay lit. Testing that it really works at those low densities before flying is a really good idea.