An outlier is a value that is outside the expected limits of the data. It Is usually more than 3 standard deviation from the mean.
In the league A, we see an outlier in the minimum value (26) and, in the league B, the maximum value (256).
The outliers can be spotted in the graph by looking at points does seem to be too far of the group of data. Also, mathematically, looking at the distance from the mean: if the difference between the value and the mean is more than 3 standard deviations, we are talking about an outlier.