What is 18 gray in photography? How does D90 adjust the white balance manually?

It should be said that the gray scale is 18%, and the gray scale of white balance calibration is 18%, but there is also white paper, and even the color temperature is directly calibrated with the back of the hand. Manual adjustment is: press and hold the button WB to be the question mark, then turn it to dial K, and then turn the front wheel to set the value. What is calibrated with the gray card is not adjusted to K, but to PRE. There are specific operations in the manual, and the steps are not clearly remembered.