In software, UsabilityTesting usually means getting users to try out the software and see what things trip them up.
This information is used to make the UserInterface better from the users' perspective.
UsabilityTesting is also used in non-software product development. For example, medical devices and airplane cockpits have been tested in this way for decades...now consumer product firms are using this usability engineering method to test all kinds of products.
See ProductTesting