XMP, or A-XMP, or some other variance is a profile that can be enabled in the BIOS to run your RAM at faster speeds.
If you buy a kit of DDR4 @ 3000MHz.... when you connect them and boot the system, they'll run at the 'standard' 2133MHz by default. You'd enable XMP (or whatever it's called by your specific motherboard manufacturer) to run at the rated speeds. Should be a relatively straightforward process. "Enable" and save/exit your BIOS.