I'm not familiar with modern CPUs, but for the 6502 it just means that they aren't documented. This means that they could theoretically change in future version

I'm not familiar with modern CPUs, but for the 6502 it just means that they aren't documented. This means that they could theoretically change in future versions or other implementations of the processor, that emulators may not emulate them, and that they may not be well tested or have surprising side effects. In the 6502 they are a side effect of how the processor is designed (see the page table.com article linked elsewhere in this thread) but may be able to do more work per clock cycle