It looks at all values you provide in between a set of brackets, each separated by a comma, and returns the lowest value.
If a value is not available:
MIN_OF_ANYsimply ignores it and gives you the highest out of values that are available
MIN, before returning a result, will ask users to provide the missing value.
p is Person
This expression will give you the value of the lowest single type of personal income, out of the three types mentioned.
If the person in this case does not have any income from wealth gain, the outcome will be the lower value of the other two sources of income.