We check the status of a website by doing a HTTP GET request.
Depending on the status response of that request we determine if the site is online/offline or having issues
If the status response is in the 200 or 300 range then we determine the site is online
If the status response is in the 400 or 500 range then we determine the site might be having issues
Any other status, or a no response is determined as being offline
The number next to the icon of the status determines the number of milliseconds that it took to get an answer on the GET request.