A
compressor stall is an event which can occur inside the workings of a
turbojet or
turbofan engine. It is what is known in
technical aviation circles as a
Bad Thing, perhaps even on the order of the
dreaded flat spin.
First, what's a stall? Okay, easy answer. What? You don't know? Good heavens. Anyhow. That's done with.
Now, turbojets and their ilk contain several sets of fans. In the frontmost part of the engine, the fans are part of what is called the compressor stage, because (can you guess?) they compress incoming air into the combustion chambers of the engine behind them. So, at the front of every jet engine (except maybe those on the SR-71 Blackbird) you can clearly see a circular bladed fan.
Each blade of the fan is, in fact, a little tiny airfoil designed to convert horizontal rotation energy into perpendicular kinetic force - the force that pushes the air into the engine. If you need to know how this works, find your local fan and examine it carefully. In order for this to work, the leading edge of the fan must be 'hitting' the air within a range of proper angles of attack. Outside that range, the fan blade's shape is incorrect for the purpose, and the fan blades will stop pushing air, even though they're spinning. When this happens, bad things escalate. The airflow into the combustion chamber is dramatically reduced, leading to (at best) a reduction in thrust. At worst, it may mean flame out as the engine is starved for air.
This process is usually started when the engine (and, likely, the aircraft attached to it) is rotated far enough from the direction of flight to produce an angle at the intake higher than this proper range. Typically, it's difficult to displace the attitude of a jet aircraft in flight enough to do this without suffering all sorts of bad effects before you reach that point...but at slow speeds, and at high angle of attack (like, say, when landing or taking off) you're especially susceptible. Compressor stalls are one of the reasons that the shape of the inlet on a jet engine is a matter of some concern, because the 'normal' airflow into the engine will determine the likelihood of these events. In addition, sometimes design of the engine and/or nacelle in which it is housed can produce poor airflow in what is otherwise a 'normal' operating condition, which will increase the likelihood of a compressor stall.