Even when there is an interaction, sometimes the main effect holds up over and aboce the interaction. That is, the main effect may be there at every level of the other grouping variable, but even more strongly at some points than at others.
In this result, the main effect for arousal holds up over and above the interaction. The effect for arousal is there for both easy and hard tasks; in both cases, low arousal produces the least performance, moderate the next most, and high arousal the most. -There is still an interaction because how much high arousal produces better performance than moderate arousal is more for hard than for easy tasks.