The discount rate is a concept that refers to a tendency of a participant (or player) to take a quick gain, instead of waiting for a pay-off further down the road. Those with high discount rates are those who would say that `a bird in the hand is worth two in the tree' or words to that effect. People with low discount rates wait patiently for the payoff.

Or, as one expert puts it:

Discount Rate

A discount rate implies that the value of future payoff is less than a current payoff for the same amount. If someone offered you $100 today or $100 in ten years, you would take the money today, invest it, and have a larger sum in ten years. In the iterated prisoner's dilemma, the higher the discount rate the lower the value of the future; this implies that the higher the discount rate the less likely cooperation will emerge.

This means that people with high discount rates don't cooperate well, and are therefore bad types.