The action is what happens when a routine gets triggered because it detected motion.

You can decide to apply a light mode, and the default is a flashing Alarm light mode for the preconfigured default monitoring routines. 
Note that some lights or devices may not be able to do the light mode you select. For example if you select "Ocean", then the lamps which do not have color capabilities will not turn on, as they cannot do "Ocean".

You can decide to not have any reaction from the lights. this can be a simple way to have presence simulation running, get notified when something is detected, to record or go check what is happening without necessarily informing the potential intruders that they may have been spotted, or to not have to de-escalate lights after a reaction.