I searched the net but still understand what it is. Can someone explain it to me?
The HTTPservice is a Service which many programs not just Roblox use to communicate over the internet. Some of the most common things made with this service is Webhooks.
To turn it on do the following:
IN YOUR GAME:
PRESS GAME SETTINGS ON THE HOME TAB
ENABLE HTTPSERVICE IN THE OPTIONS TAB
You can read more here : https://developer.roblox.com/en-us/api-reference/class/HttpService