You most likely have the IS7121 model system,
assuming so, that message means that the battery in the doorbell camera unit is low. That camera is really designed to run off of household power, through the wiring that went to your prior hardwired door bell. It does however also contain a backup battery, or can run off batteries if wiring is not available, though it wouldn't last long that way. The door camera simply uses 2 standard AA batteries.
You'll need to remove the camera from the wall mount, and there is a screw on the bottom to remove allowing access to the battery cover, then you can install 2 new batteries (lithium batteries are recommended, because they last longer), and reinstall the cover and camera.
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