certain malware can use 1 or more hardware exploit(s) to access protected kernel memory. this memory holds things like passwords, encryption keys, https session keys (banking sites ,can decode your user/pass with said keys), and so forth.
it does so by exploiting speculative execution, which is where the cpu uses “a crystal ball” to guess what to execute next while a certain procedure is waiting.
there are a total of 3 exploits, the worst of which is spectre , which seemingly affects intel cpus only. the other 2 affect all cpus , including amd and arm, but are much less serious than spectre.
Level1Techs thread on the subject:
ms has a partial patch out, and will be updating it soon. linux/mac will need to update as soon as patches become available. we dont know if android/ios is affected, as ARM hasn’t issued a statement to date. typical ARM devices are cell phones, tablets, and some laptops. other uses for ARM are rare such as servers and desktops.
so basically keep your devices updated. desktop/laptops might even update bios code as that becomes available.
correction; meltdown appears to be intel only, while spectre appears to affect all processors.