It looks at all values you provide in between a set of brackets, each separated by a comma, and returns the highest value.
If a value is not available:
MAX_OF_ANYsimply ignores it and gives you the highest out of values that are available
MAX, before returning a result, will alert you to go back and provide the missing value.
MAX_OF_ANYcan also return the highest value of a list of values provided by another function.
p is Person
If the person in this case does not have any income from wealth gain, the outcome will be the higher value of the other two sources of income.