Progressive Enhancement is a mindset to provide a basic but well-working solution first, and improve in typically small steps towards a more and more advanced solution.
In Web design progressive enhancement is seen as a “back to the roots” mentality, which favors supporting low-fidelity devices and browsers over assuming high-end hard- and software, in cycles of adding features during Web development. The idea is that well-working solutions in environments with low resources will also work fine in more advanced environments, while enhancements for the latter shouldn’t put a burden on the former. Filtering out unsuitable advancements is usually achieved by help of purposely “incompatible” import mechanisms.
A specific form of progressive enhancement is practiced in Scrum in the form of product increments, which are produced in each sprint to continuously add value to a working piece of software.