The idea behind autotagging is that you don't have to enter all of the time manually. For example a visit to facebook.com is most likely always Surfing, similarly using Outlook can always be Email. These rules may be different for each of us, but the fact that we repeat certain tasks each day is not.
To add Autotag timeline, right click on some activity you want to autotag and choose Add to autotag. In the image below, I'm adding all Firefox usage to autotag "Browsing"
Take a look at this video for a more visual explanation.
If there is no autotag timeline, ManicTime will offer to create one for you.
You can also add Autotag timeline manually. Go to Timeline editor -> Add timeline and choose AutoTag type.