This tool is used as a way to optimize your sand simulations. It will only activate particles near animated geometry, so that only a small part of your simulation is actually running at any give time. You would typically use this tool whenever you're running a sand simulation where only a small portion of the sand will be directly affected by your animation object. For example, if you have a character walking across the desert.
The initial setup occurs in SOPs where the animated object is converted to a volume and then expanded. The expanded volume is used to activate the sand nearby the animated object. In DOPs, a POP Awaken node will be added, with the Awaken by Volume checkbox turned on.
Using Awaken By Geometry
Select the object that will awaken the particles.
Click the Awaken By Geometry tool on the Grains tab.
The Offset parameter on the VDB Reshape SDF SOP controls how far away from the animated object the sand will be activated.
The green areas show particles that are awake, and the red areas show particles that are sleeping.