What is stepper motor binding? (When belts are too tight)
I think the RepRap wiki is using the word "binding", which translates to "stick together or cause to stick together in a single mass" (from Google dictionary), to indicate that some sort of friction is experienced (as you experience when things are sticking together).

When there is too much tension in the belt, pulleys and bearings experience a larger radial force stressing the balls of the bearings and pulley shafts. This causes extra friction for the stepper motor to overcome (as the friction force, tangential, is related to the radial force); this means that the stepper has to work harder and can skip steps (for more insight please read below).



While ball bearings are used to reduce friction (opposed to a bush bearing), each ball has a little friction from a couple of sources according to this reference:


  The sources of this friction are: slight deformation of the rolling elements and raceways under load, sliding friction of the rolling elements against the cage and guiding surfaces. 


These effects are generally captured in a single friction coefficient called "&mu;". The relation between friction force (tangential) and bearing loading (radial) is written by $$P_{friction}=P_{load} \times \mu$$ so the higher the belt tension ($P_{load}$), the higher the frictional force ($P_{friction}$), the harder the stepper has to work.