Improving the documentation for Extractor Objects

Author: Ashish Aneja

tutorials/tour/extractor-objects.md

@@ -10,7 +10,7 @@ tutorial-next: sequence-comprehensions
 tutorial-previous: regular-expression-patterns
 ---
 
-In Scala, patterns can be defined independently of case classes. To this end, a method named unapply is defined to yield a so-called extractor. For instance, the following code defines an extractor [object](singleton-objects.html) Twice.
+In Scala, patterns can be defined independently of case classes. To this end, a method named unapply is defined to yield a so-called extractor. An extractor can be thought of as a special method that reverses the effect of applying a particular object on some inputs. Its purpose is to 'extract' the inputs that were present before the 'apply' operation. For instance, the following code defines an extractor [object](singleton-objects.html) Twice.
 
     object Twice {
       def apply(x: Int): Int = x * 2