The trainer difficulty setting affects the resistance you feel in your trainer. For example, if you're on a slope of 10% and the trainer difficulty is at 100%, we'll send 10% slope to your trainer to simulate. If the trainer difficulty is at 50% then we'll send half of the slope, so the trainer will simulate only 5% slope.
Trainer difficulty has no impact on speed, the amount of effort you have to put out to generate that power will be the same if it was at set at 10% or 100%, although you will use different gears.
The default trainer difficulty in RGT is set at 50%.