There are also technical explanations that are similar. The result is that the energy in some system gets smaller. The semiconductor will lose energy by emitting a photon. The system will slowly lose energy because the oscillation is damped by frictional losses. Over time the pendulum will lose energy & not swing as high as before.

"Lose energy" means like "To run out of power" ex:) My battery is losing energy.. (My battery is running out of power...) If something is 'losing energy' it is mostly an electronic device and is about to run out of power. I can't explain it better :P