The threat cap is the point when any more damage caused by a dps will produce enough threat to pull aggro off of the tank. This point is the threat cap because the dps is essentially required to stop producing threat, and thus to stop doing dps if their threat-reducing ability is not available or is resisted. Since they can not risk pulling aggro from the tank, they can't do any more damage until the tank is allowed to build more threat on the mob, so reaching the threat cap without any threat-reducing ability available will greatly limit the amount of damage you can do.