Usability is the ease with which a product can be used by people. For a software product: smooth navigation through User Interfaces (UI), ease of workflow and design of the product gives the best satisfaction to the target audience.
Usability testing is evaluating product from the users' perspective. The task is to identify the problems in the User Interface and workflow (of the product), which makes users' job difficult while working on the product. If a user faces difficulty while performing a task, it means the usability has some issues that need to be fixed. These issues occur because the developers and designers of the product have limited understanding of the end user's perspective. In product companies, there is a separate usability team that takes care of the ease of usage of the product for the desired end user.