The talk at the FOSDEM conference in 2023 was about mutation testing, a technique used to test the quality of software tests. The speakers discussed the importance of writing good tests, the benefits of mutation testing, and how it can help detect bugs in code. They also talked about different frameworks available for mutation testing, such as Mutmathe, Cosmic Ray, Striker, and PyTest. They explained how to get started with mutation testing, including running tests locally and on a CI system. They emphasized the need to start small and gradually expand the testing process. Overall, the talk aimed to show developers how mutation testing is a valuable tool for improving code quality.