Isaac Lyman:

Meltdown and Spectre are the two latest exploits throwing the tech world for a loop. They have a lot in common with each other; both depend on built-in features of your computer’s processor.

After doing some reading, I think I understand the Meltdown exploit well enough to explain it in layman’s terms.

This is a well-written explainer on what happens with Meltdown. Even I understood it. And I never say this but – read the comments as well.