Try this one, I found it easier to at least think I understand what it’s talking about: quantum-munich.de/…/Negative-Absolute-Temperature…

Basic attempt at eli16: Temperatures are defined by entropy, rather than kinetic motion like we’re used to thinking. In certain constrained systems, it’s possible to create a situation where there is a maximum energy state, and saturating the particles in the system such that they’re all close to that state creates a situation where the entropy starts decreasing (the system is less disordered since all particles are at the same maximum energy). That state where the entropy is decreasing is where negative temperatures exist.

End attempt. Disclaimer, I’m probably wrong, having spent just a few minutes skimming these two articles and trying to summarize what I understood.