DER is a term used in the energy industry that stands for Distributed Energy Resources. This is a broad term that describes solutions that are generating, storing, and/or managing power in highly localized areas versus the traditional model of large power plants sending power over long transmission lines and generating from far away.
Backup is considered a "DER" inside this ecosystem as it plays the role of providing hyperlocal energy storage inside one's home. Furthermore, Backup is inside the category of "Permissionless DERs" meaning you can install and deploy them without any cumbersome permitting, rewiring, or renovation. You can just do it and get going, it all lives in front of your wall outlet.