This is an easy one. For 99% of your workouts you do not want to use ERG mode.
There are two modes in which you can run a structured workout: ERG mode or incline mode.
In ERG mode the smart trainer essentially feeds you the wattage that is programmed into the structured workout. Low cadence, high cadence it doesn't matter. The trainer will feed you the same wattage - you can't escape the wattage. Resistsance is futile.
The only workout where you might want to use ERG mode is on a step test. But even then, as you develop more control of your wattage, you will find you can do it on incline mode and it will feel much better.
The other option is to use incline mode, where the trainer creates a steady incline/gradient that you work against. When riding against the incline your cadence, gearing and torque will influence the wattage you produce. This is more like riding outdoors.
The easiest way to switch out of ERG mode is in the Zwift Companion app. Once you have loaded the structured workout and started riding the app will go into workout mode. There you will see the option to toggle off ERG mode.
You will also have the option of turning off ERG mode when you are choosing the structured workout from the workouts folder.