Words are normally black on a white background. 
Highlighting turns backgrounds into a different color. Like this white sentence with a dark background.
Here is how to highlight:
  1. Place the tip of the arrow in front of the words to highlight.
  2. Hold the left mouse button down.
  3. Drag (move the mouse) arrow across the words.
  4. When the background behind words are a different color, they're highlighted.
Try highlighting this word. ---------------------------------> Hey,
Try highlighting this sentence. -----------------------------> Hey, I did it!
Try highlighting this paragraph. ---------------------------->
You will need to drag the arrow down as well as across.		 Hey, I did it. I highlighted this whole paragraph.
But why? What's the point of highlighting?

For the answer click 31