This voltage is set between two ends of the Hall plate. The ends are perpendicular to the specified current direction.
Hall constant is a material constant and depends on the material of the sample used. More precisely: It depends on the charge carrier density of the Hall plate and the polarity of the charge carriers.
Electric current is the number of charges per second that pass through the Hall sample (between two ends).
Magnetic flux density tells how strong the external magnetic field is, which is applied perpendicular to the Hall sample (and thus to the current direction).
Thickness of the sample in which the Hall effect is studied. This can be, for example, the thickness of a rectangular metal plate.