A set of gears are employed to transmit motion from the main shaft of the machine to various revolving elements. A combination of gears employed to transmit motion from one shaft to other(s) is called Gear train.A gear train is a mechanical system formed by mounting gears on a frame so the teeth of the gears engage. Gear teeth are designed to ensure the pitch circles of engaging gears roll on each other without slipping, providing a smooth transmission of rotation from one gear to the next.
In other words, it’s an assembly of gears connected to each other that changes the rotational speed of what is providing power versus what is receiving it. Usually but not always, the output end of a gear train is slower, which also multiplies the torque (power). An example would be a boat trailer winch; the guy turning the hand crank runs a series of gears that slow the spool with the cable to a small fraction of the handle speed. It allows a skinny guy like me to haul a ton or more of the boat up and onto its trailer; slow but powerful.
A more complex gear train exists in an old watch, where the tick-tock escapement gear is very fast, but through gears is slowed to 2-3 different speeds, depending upon whether the watch has a sweep second hand. The other two drive the minute and hour hand, by tapping rotation speed through various gears in the train. Hope this explains something.