Hx refers to the horizontal difference between the center of the bottom bracket to the center of the handlebar bore.  This measurement is taken horizontal to the ground.

Hy refers to the vertical difference between the center of the bottom bracket and the center of the handlebar bore.  This measurement is taken perpendicular to the ground

 

Hx and Hy as measured on a road bike.
Hx and Hy as measured on a road bike.

 

Hx and Hy as measured on a triathlon bike.
Hx and Hy as measured on a triathlon bike.