WOT is Web of Trust, a free, popular browser add-on I’ve used for a long time. It acts as a website checker, notifying you about each site’s reputation. It uses a colour-coded icon alongside the site name, as here:
The codes are green, yellow, and red, while grey indicates no rating:
If you hover your cursor over the icon, you can see the rating:
You can view the full scorecard and add your own rating if you wish.