It's more likely than Y the probability that they're going to choose right is more than Y, then the highest line of these three is this one, which corresponds to my choosing Middle.

即比Y大,也就说对手选右的概率大于Y,那么这三条线中最高的是这条线,这条线表示我选中的收益

耶鲁公开课 - 博弈论课程节选