What a Usability Test is
“Usability is a quality attribute that assesses how easy user interfaces are to use. The term also refers to methods for improving ease-of-use during the design process.”  (Jakob Nielsen)
A usability test is a method of evaluating a product by testing it on users. Nowadays it is most applied in an informal/guerrilla-like method in an environment controlled in a minimal way. As most user researches the statistical significance is not taken into account. Instead it’s used as a method of broadening perspective and inspiring design. Usability testing are conducted with a group of potential or current users. They are asked to complete a series of typical tasks. Usually tests are recorded and analyzed to identify areas of product improvement. It is known that 5 users are a sufficient sample size to inform about the performance of the interface  .