its nothing but a how the user can able to interact with 
the application.

Usability testing is a technique used to evaluate a product
by testing it on users. This can be seen as an irreplaceable
usability practice, since it gives direct input on how real
users use the system. This is in contrast with usability
inspection methods where experts use different methods to
evaluate a user interface without involving users.
Usability testing focuses on measuring a human-made
product's capacity to meet its intended purpose. Examples of
products that commonly benefit from usability testing are
web sites or web applications, computer interfaces,
documents, or devices.

-anjanitesting@gmail.com