There are 4 statuses that will show for an agent on the HUB: Active, Pending, Never Connected, and Disconnected. Below are the explanations for each status
Active: The agent is connected and the HUB is receiving alerts
Pending: This is a transitional state, the agent is in the process of connecting.
Never Connected: This means agent registered (TCP 1515) but has never connected over remoted (UDP/TCP 1514)
Disconnected: Agent registered (TCP 1515) and had previously connected over remoted (TCP/UDP 1514) but is no longer online. See here for troubleshooting