The "Waiting for Approval" tab under the "Computers" view lists out all the computers in which the agent has been installed but still are queued for the system administrator to approve. Only after this approval any further communication with the Central Server will be established. The computers that are approved in this queue will move to "Managed Computers" to which all the server configurations will be applicable.
Supposedly, the agent gets installed in a client computer without the knowledge of the system administrator, this feature will ensure that further communication with the server will only be established after approval. The computers waiting for approval in the queue will still have the agent installed, but all the status updates will be rejected by the server.
You can activate the approval setting by enabling it in the web console, go to Agent -> Agent settings -> Actions to be performed after agent installation -> Enable "Waiting for Approval option. Unless you enable this option all the computers in which the agent is installed will be moved under Managed Computers by default.
Once you enable this setting, Go to Computers -> Waiting for Approval and select computers to approve or decline. When declined, further communication with the server will be terminated.
The Waiting for Approval tab will be shown under Computers only when the "Waiting for Approval" option in Agent settings has been enabled.