Drain the mail queues on the server.

I want to put the Exchange 2016 server into maintenance mode before rebooting it. The first command is to  Drain the mail queues on the server.  

Set-ServerComponentState EX16-1 -Component HubTransport -State Draining -Requester Maintenance

Question:  What does it mean by draining the mail queues.  What would this command do on server? I want to understand it practically . Can some one explain please.