The server and monitor produce heat.
The server has one or more fans that pull in fresh air and force out hot air.
The monitor lets hot air escape through vents.
Blocking the air vents can cause overheating, which might result in a malfunction or damage.
Place the server and monitor so that nothing blocks the air vents; usually, 15 cm (6 inches) of air space is sufficient.
Also, make sure the vented air is not blowing on someone else.