The efficiency of a machine is a how the input energy is converted in output work
Then in the middle you have things consuming the input energy and not going to the output work, the best you can do is reducre those things consuming, for a machine it can be friction, heat, vibration or loose parts.
Friction refers to the resistance that a surface encounters while moving on the surface of another. Machines often posses moving parts such as rollers. Those parts move on other surfaces generating friction. Friction in machine parts leads to useless work and reduces the amount of input work used by the machine to do useful work. The efficiency of a machine has to do with its ability to convert input work into output work. This efficiency is greatly reduced by friction in the moving parts of the machine. Hence friction decreases the efficiency (work done by a machine)
