…which may be used to pass in the URL of an image to download at boot, and the system will then do so, as part of the regular boot process, right after networking is available.
This is particularly useful in VM and full OS containers: you can just boot a base image, and then tell it via the two mechanisms what payload to actually run on the system.
For details see the man page:
https://www.freedesktop.org/software/systemd/man/devel/systemd-import-generator.html
www.freedesktop.orgsystemd-import-generator