I agree that I need to provide additional verbage on how to use the options and the fight model.
Holymist has it very close.
It's how much damage is incoming over any period of time. Avoidance is factored in as a % of raw damage reduction. The handle the velocity of Boss hits, is to model the expertise/parry-hasting that can happen. And given that most of the
models are setup for level 80 toons. It will not always give very accurate results for toons below that level.
The user can tune how much damage they expect to see incoming over a given period of time. Increasing this value places more value on Avoidance. Decreasing this value will decrease Avoidance values for Gear and Gems and such.