Source: https://8thlight.com/blog/uncle-bob/2012/08/13/the-clean-architecture.html

The Dependency Rule says that source code dependencies can only point inwards.
Nothing in an inner circle can know anything at all about something in an outer circle.

By separating the software into layers, and conforming to The Dependency Rule, you will create a system that is intrinsically testable, with all the benefits that implies. When any of the external parts of the system become obsolete, like the database, or the web framework, you can replace those obsolete elements with a minimum of fuss.