Well to put it simply the abbreviation KP comes from the German “Planetarische Kennziffer” which translates to planetary numbers. In English- Planetary Index. So basically- KP-index is a mix of terminology indicating your chances of witnessing the Aurora Borealis.
It’s a rising numerical function that is used to calculate and indicate the chances of conditions which are conducive to aurora borealis occurring. The higher the number- the higher the chances of you getting to see some northern lights.
The KP number is a system of measuring aurora strength. It goes from 0 to 9 (0 being very weak, 9 being a major geomagnetic storm with strong auroras visible). So when you’re looking at the aurora forecast page, you want to look for high Kp numbers. The higher the better. Anything above (and including) Kp5 is classed as a geomagnetic storm.