Edge storage records video on the camera itself, typically to an SD card, instead of only at a recorder. It keeps footage when the network or recorder fails and gives a second copy of the same event. Card size, write endurance and the recording mode set the actual retention.
The camera is the one device that sees the event directly. Edge storage takes advantage of that, keeping a copy at the point of capture.
What edge storage buys
If a cable is cut or a recorder fails, the camera can keep recording to its card, and an investigator can recover footage from the device itself. It also reduces the consequence of an outage for cameras on a long or unreliable network link.
Limits and management
Cards are small, subject to wear and easy to remove, so edge storage is not a substitute for a recorder. It can record continuously or only on an event. Someone has to check card health and include card retrieval in the response plan, or the benefit stays theoretical.