What is tranformer turn ratio?

Transformer turn ratio is the ratio between the turns on the primary side and the number of turns on the secondary side. For instance if you have a turn ration of 100:200 the primary side has 100 turns and the secondary side has 200 turns. This will be a step up transformer. 10 volts AC into the primary side will result in 20 volts on the secondary side. There will be a decrease in current but the voltage will increase. The total power into and out of the transformer will be the same minus some losses for resistance in the windings.