In Google Sheets, COMBIN is a function that calculates the number of combinations of k items that can be selected from a set of n items, without regard to order. The function takes two arguments:

n: This is the total number of items in the set. k: This is the number of items to select from the set.

The COMBIN function returns the number of possible combinations of k items that can be selected from the set of n items. The formula for the function is:

=COMBIN(n, k)

For example, if you have a set of 10 items and you want to know how many combinations of 3 items can be selected from the set, you can use the COMBIN function like this:

=COMBIN(10, 3)

The resulting number would be 120, which is the number of possible combinations of 3 items that can be selected from a set of 10 items.