JMockit is a Java-based testing framework that allows developers to easily mock or simulate dependencies within their code. With JMockit, developers can create complex test scenarios, inject mock objects into their code, and test individual units of code in isolation from the rest of their system. This helps to ensure that code is more reliable, easier to maintain, and functions as expected. JMockit is particularly useful for test-driven development (TDD), where developers can write their tests before implementing their code to ensure it meets the desired criteria.