WP is White Point. That is, the maximum meaningful value a pixel can have (in the raw file, which is basically a stream of (here 14-bit) integers. The exact value depends on the sensor, and is provided in the EXIF data (in your case, 14800 and 15300).
I’m not sure why there are two white point values, unless the lower one gives the upper limit of the linear response, and the higher gives the actual sensor saturation. (if pixels are between the two values, they have meaninful data, but colours can be off).