The Console tab in the server panel shows your server's log stream in real time. Errors are tinted red, warnings yellow and info lines green.
Use the command bar at the bottom to send any server command: e.g. 'say Hello', 'time set day', 'whitelist add Steve'. No leading / needed.
Navigate command history with the up/down arrow keys. The console also reconnects automatically when the server restarts.
While the server is stopped the console stays read-only; inspect the final shutdown logs to see why it stopped.