The document discusses software testability and defines it as the degree to which a system can be tested. It lists several factors that contribute to good testability, including operability, visibility, resolvability, simplicity, intelligibility, control, and stability. Good testability is characterized by code that is well-designed, loosely coupled, and easy to interact with and observe. Lack of testability is caused by things like tight coupling, unclear design, and lack of support from developers and management.