(Reach) Length in mm is the distance from the center of the handlebar to the center of the steerer tube. But this cannot be measured directly, you need to lay the tape measure in a position where you can estimate it. Method of measuring depends on the handlebar clamp. Eg: On Brodie, if you remove the handlebar clamp, you can then measure from the flat surface to the center of the handlebar stem.
A long one is going to put the handlebars further forward.