Definition of risk/reward
Literally, "risk-reward". If you want a reward, you have to take a risk. This is often the case in poker. The best example of the risk/reward concept is bluffing. When you bluff, you take a risk. If we bet 100€ on the river in bluff, we risk 100€. If our opponent calls our bluff, we lose $100. But if he folds, we win a pot that we wouldn't have won otherwise.
Calculate his ratio of risk/reward
When we take a risk to get a reward, we want the risk/reward ratio to be good. To use the example of bluffing that we were talking about above, if we bet €100 in bluffing to win a €5 pot, it's an inordinate risk for a minimal reward. On the other hand, if we bet €100 on a €2500 pot bluff, our bet will not discourage anyone from calling. The risk our opponent will take will not be big enough to discourage him from calling the bet. Usually you have to select a good ratio of risk/reward. If the reward is very high and the risk is acceptable, it's worth it. But if the risk is too great for the reward, or the reward is too risky for the risk, you should skip your turn.