The different ski binding manufacturers have different binding angles so
changing from one brand to another can affect your stance, posture &
balance etc. The measurement is the additional height of the heel above
the toe. 

The manufacturers often build in this height in an attempt to help users
get forward more. If you have too much forward tilt (or delta as it's correctly
known), especially when combined with the forward lean of your boots,
it can be counter productive as you end up sticking your backside out
to counterbalance.

Here's the binding delta angles for the different binding manufacturers:
Marker Duke & Baron             0mm
Atomic                                +2mm
Salomon                             +2mm
Fischer/Tyrolia                     +3mm
Look/Rossi, Marker & VIST  + 5mm         

You can check easily yourself. Snap your boots in to your bindings &
put the ski on the floor, pressing the camber down flat. Measure from
the floor to the bottom edge of your boot at the toe and the heel. Your
own specific binding/ski combination might be slightly different than
stated above as the thickness of the ski will affect relative heights.

Remember that basic geometry means that smaller boots will give a
greater delta angle than larger boots in the same binding. WC racers 
often use bindings that have no delta so it's then easy to adjust
their set-up as they require it.
 
When I had my boots correctly aligned & balanced by Colin & Andi at Solutions4feet (see the UK Boot Fitting & Alignment section) I balanced best with my toes 2mm higher than my heels. I therefore use 7mm shims under my toe bindings to offsets the +5mm delta of the Vist heel piece, as shown in the pic.