Hi Sharon,
“get storage item object” is part of nanoflow commons, documentation is available here: https://github.com/mendix/nanoflow-commons
When you're offline a nanoflow retrieve action will use the data you've stored on your device through synchronisation, more on this can be found in the mendix documentation, more on offline-first: https://docs.mendix.com/refguide/offline-first
Hopefully this helps you out.