It gives you the average of the values you provide in between a set of brackets, each separated by a comma.
If a value is not available:
AVG_OF_ANYsimply ignores it and gives you the average of values that are available
AVG, before returning a result, will alert you to go back and provide the missing value.
p is Person
If the person in this case does not have any income from wealth gain, the outcome will be the average of the other two sources of income.