Essentially this is a line which helps define horizontal plane in the skull, aka define zero degrees so that other measures can be made more accurately.
The scientific method of measuring the Frankfort Horizontal line is done with a lateral xray. It is the line which is created by the opening of the external auditory meatus and the low point on the lower margin of the left infraorbital rim.
A practical application of this is to use the top of the tragus and palpate your inferior aspect of your orbital rim. The orbital rim is essentially the cheek bone which is just below your eye. Some patients may ask why this has been declared horizontal. It is an arbitrary designation which is easy to duplicate. It helps standardize photographs and measures.