The number of ways of selecting items where the order is not important, is known as a number of

Let's consider  hmmm a group of letters, say AAABBBCCC.

now, we can combine them in groups of say 3, like ABC, or BBA, or AAB, or CBA, and so on.

notice ABC and CBA has the same letters, arranged differently, but the same, a "permutation" says, that is ok, "order IS important", meaning ABC and CBA are really two different groups, because of how they're arranged.

but a "combination" says, wait just a second here, you've got the same letters, "order is NOT important" for me, so ABC and CBA is the same cat with a different dress, come on now, you can't fool me, go get other letters you slacker.