In the simplest case, payoff means a reward or reinforcement that the system gets from the environment. It's a scalar quantity, like 37 or 764.3. An individual classifier's payoff prediction is its estimate of what the system will get if the system takes the action recommended by this classifier.

In the general case, payoff may also be an estimate of a combination of external (from the environment) reward, and various forms of internal reward that the system employs in its operation.

What do you mean by "payoff"?