INDEX(($A$2:$A$100=L1)*($B$2:$B$100=M1),)
Can someone explain what it does
It returns a vertical array of 1s and 0s.
Let's assume L1 = x and M1 = y.
......A.....B
2...x......y
3...z......z
4...x.....y
5...x.....x
(A2=L1)*(B2=M1)
(A3=L1)*(B3=M1)
(A4=L1)*(B4=M1)
(A5=L1)*(B5=M1)
TRUE*TRUE = 1
FALSE*FALSE = 0
TRUE*TRUE = 1
FALSE*FALSE = 0
INDEX({1;0;1;0},)
The comma means the row_num argument has been omitted so it defaults to 0
which means to return the entire array. Why the formula is written that way
depends on how it's being used.