A value will be excluded from a rational expression if it causes the denominator to be zero as dividing by zero is undefined.
An example that would work for your specific question is:
If you plug in either 2 or 5 into this equation the denominator will be zero causing the expression to undefined there, so the values 2 and 5 are excluded from the domain of the expression.
If data is skewed then the upper and lower half have different amount of spread. Here there is no proof to say that data is not spread around mean or there are outliers. Therefore we can't say that the distribution of wings dinners sales in not normal.