The idle step defines what the lamps do when you activate the routine and afterwards, while no motion gets detected.

In this step, you can decide to apply a light mode, which will be applied at routine start. 
Note that the same light mode will be also applied if your "reset" the routine once it has been triggered.

You can decide to not apply any light mode, meaning that the lights will not change from what they were doing before you activated the routine. Note that in this case, when you reset the routine if it has been triggered, no light mode would get applied to the lights.

You can decide to use the presence simulation functionality which will not apply directly any light mode to the lights, but will replace the scheduling of those lights by some standard ones to simulate presence, turning the lights on and the off in the morning... and then on and off in the evening at set times. This will also deactivate your normal scheduling to make sure it does not trigger any permanent on state for example.