Introduction
If you have a MAC Book /Pro/Air laptop, then you can tap only on the trackpad instead of clicking to perform a left click. This feature is not enabled by default.
Enabling Tap to click is a simple operation :
1. Open System Preferences. Do this by clicking on the Apple logo
  in the top left side of the screen and then click on System Preferences…
2. In System Preferences and under the Hardware section, click on Trackpad
3. Under the Point & Click tab, you can find the first option, Tap to click, unselected by default. Select this option, and you will instantly be able to tap to click.
start tapping with one finger and you will see that you don’t need to click anymore on the trackpad.
Summary
Tap to click is an option that is available in MAC OS X trackpad but is not enabled by default.