NUnit is a unit testing framework for .NET languages that allows developers to write and run automated tests to determine if sections of code are functioning as intended. It works by defining TestFixtures containing individual Test methods annotated with attributes to identify setup, teardown, and expected exceptions. The presentation introduces unit testing and NUnit, explaining that unit testing verifies code modules are fit for use and NUnit is a widely used open source unit testing framework written in C# that brings the benefits of xUnit testing to .NET projects.