Back to von Boehmer's data. What his observation showed was that if your T cell cannot recognize the MHC expressed by the thymus it will not mature and will be stuck at the DP stage. It turns out that cells that cannot recognize thymic MHC ultimately die at the DP stage and this cell loss contributes most significantly to the cell loss observed by the scientists who looked at developmental kinetics. These are important concepts that we will flesh out in the future. The lower the Kd the lower the affinity.
Avidity is a very useful concept that has very vague and controversial definitions.
I think of it as a collective 'strength' of physiological interactions that takes into account multi-valencies AND other affinities by other coactivating, costimulating molecules.