An auxiliary power unit, or APU, is a small gas turbine generator
usually located in the tail
section of a jet aircraft
. The telltale sign of an APU is a little exhaust port at the rear end of the aircraft's fuselage
, and often an intake
on the leading edge
of the tail fin
A jet aircraft has an APU because during flight, its pressurization, radio, and other systems are powered by generators connected to its engines. When the engines are turned off on the ground, the APU kicks in to power the aircraft's vital systems.
If the lights on an aircraft shut down momentarily just after arrival, it's probably because the APU hasn't had enough time to warm up.