Meltdown – on Intel CPUs and the Arm Cortex-A75 – allows normal applications to read protected kernel memory, allowing them to steal passwords and other secrets. It is easy to exploit, but easy to patch – and workarounds to kill the vulnerability are available for Windows and Linux, and are already in macOS High Sierra, for Intel parts. There are Linux kernel patches available for the Cortex-A75.
There's also another security flaw named
Spectre that affects, to varying degrees, Intel, AMD, and Arm.
Depending on your CPU, Spectre allows normal apps to potentially steal information from other apps, the kernel, or the underlying hypervisor. Spectre is difficult to exploit, but also difficult to fully patch – and is going to be the real stinger from all of this.