Problem Spring Web is an implementation of the proposed
application/problem+json HTTP API problem details response for Spring Web (im both its MVC and WebFlux flavours).
Error responses should be a first-class citizen when it comes to providing REST APIs. These responses often are generated in an ad-hoc, non-standardised fashion, which not only makes handling such responses more difficult and tedious in client applications but also makes for less maintainable software in the back-end software providing the API.
Problem Spring Web attempts to solve these issues for Spring applications by building on a defined standard for error responses.
These articles go into more detail as to best practices for handling REST APIs in general and how to use the Problem Spring Web library, specifically.