It means Xtreme Memory Profile. What XMP does is, it allow your Motherboard BIOS to automatically configure the overclocked DRAM voltage and timing information instead of forcing you to do it manually. Though it does not guarantee stability.
You can use XMP RAM in non-XMP motherboards and you can use non-XMP RAM in XMP motherboards. In both cases you will simply have to set the timings and voltage manually if you decide to go outside of the recommended specifications.