As many are aware, there were a few CPU vulnerabilities codenamed Spectre and Meltdown. While the Meltdown vulnerability targets INTEL CPUs, the Spectre vulnerability affects many CPUs (AMD, INTEL, ARM, etc.).
The first article explains these vulnerabilities very well. An overview of these vulnerabilities is modern CPUs are designed to predict what you want to do, so it queues it up. Bad actors can exploit this by running code that won't finish, but would be part of the prediction the CPU does. This code would then be able to output an answer that is run in the actual code. This answer could contain parts of sensitive information, like your password.