He is using the Right() function to take the 2 characters off the end.
So if his month is january the Month() function will return 1 .... so "0" & 1 = "01"
And his month is december the Month() function will return 12 ... so "0" & 12 = "012"
By taking the 2 rightmost character he will always have a two character string.... the leading zero gets chopped off for October, November, & December (010, 011, 012)