In simple terms with caveats and nuances ignored, at times there can be so much power being produced during times of low demand that the electric companies / systems will lower prices on electricity to incentivize its use. In some cases, the amount of power being generated exceeds demand so much that prices can go negative, and they essentially pay others to use it.
There are many reasons for it, and I’m hardly an expert, but essentially some power sources are unpredictable or unstable (ex: solar power is only generating during the day) and others can’t be shut down quickly/easily in response to demand. So, this is one way to deal with excess electricity production.