I think the statuses just match the ones listed in the main documentation...
https://documentation.meraki.com/General_Administration/Cross-Platform_Content/Status_Icon_Meanings
...from that page:
Every device can display in one of four colors, indicating the status of that device:
Green: The Cisco Meraki device is currently online and has no alerts
Yellow: The Cisco Meraki device is currently online but has one or more active alerts
Red: The Cisco Meraki device is currently offline and is inaccessible by the Cisco Meraki Dashboard
Grey: The Cisco Meraki device is currently offline and has been offline for at least a week (dormant).
These indicators can be a quick and easy way to narrow down what sort of trouble a device is experiencing. A device indicating it is offline likely cannot reach it's gateway or Dashboard. A device indicating a yellow alerting status will often have additional details on the device details page. Examples include IP address conflicts or disconnection from RADIUS.