What is the X in X-mas?
The eight keys on a typewriter or computer keyboard on which a touch-typist positions their eight finger tips when starting to type or when resting between words or phrases. Typists learn the position of all keys on the keyboard in relation to the home keys.
On a standard english keyboard layout, the home keys are ASDF for the left hand and JKL; for the right. Most keyboards have small raised bumps on the left and right index finger keys (F and J) so you can find the home keys by touch without looking.