It's a POE input. You can use it to power the switch instead of the AC adapter.
Judging by your original post you're looking at devices that cost far more than you need to spend. You just want a basic unmanaged Gigabit switch (usually under £20).
Ya I hear you. I was just thinking "Out of the Box" as the Power AC adapter is short and the switch is not going to be close to an electrical outlet
so I was thinking of using the POE socket via the router. Anyway I have now had a total rethink as you are right, my needs are minimal I have gone for the TP-Link https://www.overclockers.co.uk/tp-link-5-port-gigabit-easy-smart-switch-tl-sg105e-nw-134-tp.html
.