Reducing Coupling Through Unit Tests
Low coupling refers to a relationship in which one module interacts with another module through a stable interface and does not need to be concerned with the other module's internal implementation.
Unit testing under any name is a good test of the ability to call your code in isolation.
If you have written code with low coupling, it should be easy to unit test. If you have written code with high degrees of coupling, you're likely to be in for a world of pain as you try to shoehorn the code into your test harness.
Is the code highly cohesive? That is, does each module carry a single, reasonably simple responsibility, and is all the code with the same responsibility combined in a single module? If code implementing a single feature of your application is littered all over the place, or if your methods and classes try to do many different things, you almost invariable end up with a lot of coupling between them, so code with low cohesion is a big red flag alerting you to the likelihood of high coupling as well.