The Meltdown and Spectre chip flaws take advantage of a modern CPU performance feature called speculative execution..

Speculative execution improves speed by operating on multiple instructions at once—possibly in a different order than when they entered the CPU.
To increase performance, the CPU predicts which path of a branch is most likely to be taken, and will speculatively continue execution down that path even before the branch is completed. If the prediction was wrong, this speculative execution is rolled back in a way that is intended to be invisible to software..

The Meltdown and Spectre exploitation techniques abuse speculative execution to access privileged memory—including that of the kernel—from a less-privileged user process such as a malicious app running on a device.. ..Read More »

The Meltdown flaw affects laptops, desktop computers and internet servers with Intel chips. It lets hackers bypass the hardware barrier between applications run by users and the computer's kernel memory..
https://www.nairaland.com/4270563/meltdown-security-flaws-spectre-bugs