Eustress would be, in simple terms, "good stress." Stress usually follows an inverted "U" curve. Very little stress means I feel bored and I don't feel like stretching myself. So stress leading me to stretch myself is - Eustress. But, that which is beyond the peak of "U" would be debilitating.
There is also another viewpoint that the conceptualization of the inverted "U" may not be correct, and beyond the peak, there can be a plateau. This would apply to high achievers who love to chase a moving target.
Related to games, I use running a 100m stretch. You measure the exact time each participant takes. Then another round, with measurement. One more if participants are not tired but even asking them to imagine one more round would help explain the inverted "U." It is invigorating to do a 100m run the first time. It brings us out of lethargy, etc. But doing it again and again improves performance until some point. Then tiredness does set in. (The levels may be different for different people)