The UAV Status widget shows you an overall summary of the states of the UAVs in the swarm. UAVs are sorted into four non-overlapping sets:
The first, green group corresponds to UAVs without any problem.
The second, blue group corresponds to UAVs with an info-level error code (e.g. initializing).
The third, yellow group corresponds to UAVs with a warning-level error code (e.g., low battery).
The fourth, red group corresponds to UAVs with severe/critical errors (e.g. GPS outage or compass error).
If you move your cursor over the widget, you get a tooltip that lists the physical IDs of all UAVs and the overall number of UAVs in a particular state, sorted by severity of states and error codes. Clicking on the widget itself reveals the UAVs panel of the main workbench area. Clicking on a row in the tooltip selects all UAVs in the corresponding state.
The list of available error codes of all severity levels from informational messages to critical errors can be found in the public online Skybrush protocol specification.