in a VLOOKUP formula, what does the argument column$\_$index$\_$number mean?

This is the secret INDEX number that each column in an Excel worksheet has.

This is numbers you assign to columns in your worksheet.

This is the number of the column within the selected table$\_$array where the lookup result is located.

This is a numerical representation of the letters at the top of a worksheet. For example, $A=1,B=2,$ etc.