Monitoring Routines are a way to change the way the whole home reacts to motion detection.
You can define what you want to happen when you activate the routine (this is also what will apply if you reset the routine). This can be to turn off the lights for example, to make sure everything if off when you have left. This can also be to apply a presence simulation functionality automatically turning on your lights in the morning, then off after some time, and on again in the evening, then off again, simulating a presence at home while you are far away (instead of your normal schedules).
Then, you can select as part of the routine which available sensors in your home to take into consideration to assess movement. This can be a mix of cameras being armed, SpaceSense being activated in some of the rooms, and even motion sensors you may have set up.
Finally, you can configure what you want the answer to the detection of movement to be. It could be to turn on the lights at full brightness, or to flash the lights with the new alarm light mode to deter intruders and visually inform neighbours... or to not do anything.
Note that when an event is detected, if you have the notification activated for this type of event, you will receive it on your mobile phone, informing you that the routine detected something, and it should have triggered the reaction defined.
You can also stop the active routine in order to have the Home go back to its standard settings of automation.
If the monitoring routine is in a triggered state, having detected movement, you can also reset it to re-apply the initial routine state, same as you were to stop it and restart it.