If you were to slide an carriage along it's axis by hand and were to feel a position on that axis where it is harder to push the carriage you would say that it is binding at that point. Binding may cause a carriage to stop moving thus loosing steps and upsetting your cut.
Racking usually applies to the Y axis and is where one end of the axis moves while the other end stays still. Imagine that you have a stepper on each side of the Y axis and one motor stops turning for what ever reason ,(maybe binding ), but the other motor keeps going. The Y axis is now racked and will no longer be square to the X axis. A center drive Y axis can also rack if the router is cutting on the left side of the axis it will have a lot of resistance while the right hand side will have none. This will cause the axis to twist or rack.
Hope my long winded explanations make sense.