This happens when your tracker hasn't checked in with our servers for more than 10 minutes. The most likely cause is that it doesn't have power or cellular/mobile reception (especially if located indoors or underground).
AT1 - Portable GPS tracker specific
If your AT1 portable tracker is reporting "Sleeping" please see FAQ on battery modes:
https://shieldgps.zendesk.com/hc/en-us/articles/26152989098644--What-are-my-AT1-battery-modes