The unit philosophers use to quantify pleasure. Hedons are abstract units with no standardized metric: the assignment of hedons to experiences is typically very broad, loose, subjective, and case dependent. Negative hedons can stand for whatever detracts from a pleasurable experience, or for pain -- though sometimes the unit used for pain is the "dolor".
For example, we might say that eating an ice cream cone yields 3 hedons to a normal individual, while solving an intensely difficult logic puzzle yields 15 hedons. The frustration undergone in the course of figuring out the puzzle might be judged as -20 hedons (or 20 dolors), depending on the puzzle's difficulty and the individual's temperament. The specific numbers chosen are generally not too important; the main point is to give a rough sketch of how the enjoyment of one experience relates to others.
Utilitarians often employ hedons to compare the expected net outcome of pleasure for a given scenario. Hedonism is an ethical theory that sees the maximization of hedons (and the minimization of dolors) as the most desirable good.