'.this'

The makeText() method of the Toast class expects a Context object as the first parameter. Since the Activity class is a subclass of the Context class you can pass a reference to the current activity, which is the FunFactsActivity. The way you get a reference to the current activity instance is by using the this keyword. We could have just used this instead of FunFactsActivity.this. However, there are cases where you need to use the name of the class. For example, if the line of code for creating the toast was inside the onClick method then using this would refer to the instance of the anonymous listener inner class and not the activity instance. In that case, we have to use FunFactsActivity.this.

.this keyword means,that you are calling a field of the class.For example:

class Worker{
   public String name;

   public Worker(string name){
      this.name = name;
   }
}