It’s Spectre/Meltdown. Basically the CPU will guess at whether a conditional (“if…then”) will come true or not, which boosts performance quite a bit by letting the CPU continue to work while other parts of it are figuring out the conditional. It’s called branch prediction. The branch that doesn’t “win” is supposed to get flushed, but you can be lazy and skip this step, since the leftover data will just be overwritten anyway. Except until that happens, the data can still be read. Meltdown was similar and used a race condition to let code read any memory location in the entire system, which is a disastrous vulnerability since you could sniff out administrator or encryption keys that way. I’m not a computer scientist, I can’t really explain it all because I only understand the basics of it myself.
Anyway these are not a big deal if you’re sitting at home, any software that could sniff data this way might as well just issue a read to the file system and get tonnes more data with much less effort that way, but in a datacentre, where dozens of customers may be using a single processor at once in a VPS, I could sniff out your data from my share. It breaks memory protection barriers between containers/virtual machines. That’s catastrophic, considering how important VPS services like Azure and AWS are. Software fixes could be implemented, but affected Intel processors (something like 2011-2019) would be slowed down up to 30%. It was a huge hit to Intel’s competitiveness and credibility.