A gear train is a sequence of gears and axles mechanically connecting an input axle to an output axle. This can be any number of axles and therefore creates a "train" of gears. Usually each intermediary axle has two gears on it - one being driven by the previous axle's gear and one that drives the next axle in the series. The diagram below shows and example of a gear train. On the left is the input axle and this drives the black 12 tooth gear onto the tan 20 tooth gear which is connected to the 2nd axle. This axle then drives the grey 16 tooth gear onto the next 16 tooth gear (on the third axles) and that axle in turn has an 8 tooth gear driving the 24 tooth gear on the output axle (on the right). The overall gearing ratio between the input axle on the left and the output axle on the right is simply the product of all of the individual gearing ratios in the gear train. So that gives:

Overall Gearing Ratio =  (12 / 20) x (16 / 16) x (8 / 24) = 1 / 5 or 0.2.

This means the output axle rotates one revolution for every 5 revolutions of the input axle, or simply that the output axle is 1/5 the speed of the input axle.