I don't understand how to use $scope.$watch and $scope.$apply. The official documentation isn't helpful.

What I don't understand specifically:

• Are they connected to the DOM?
• How can I update DOM changes to the model?
• What is the connection point between them?

I tried this tutorial, but it takes the understanding of $watch and $apply for granted.

What do $apply and $watch do, and how do I use them appropriately?