In a mechanical process, parts move and do work. A car engine, a bicycle chain and a lift motor all transfer energy to make something move. The useful output is the movement, but not all of the energy ends up there.
Whenever moving surfaces rub together, there is friction. Friction makes the surfaces and the air around them warmer, so some of the energy is transferred by heating. A rise in temperature is the sign that energy is being wasted.
The warm parts then heat the surroundings, such as the air, because they are hotter than their environment. The energy spreads out into the surroundings. This is called dissipation, and the energy cannot easily be gathered up and used again. Any process that causes a rise in temperature is therefore wasteful, because energy has been dissipated in heating the surroundings.
The total amount of energy is still conserved. The energy has not vanished. It has been transferred to the thermal store of the surroundings, where it is no longer useful.