Apostrophe s. It has two purposes. 1. Possessive. 2. Contraction.
In the Possessive form, it means "belongs to". Mike's fake glasses belong to Mike. As a Contraction, it adds "is" to the noun it follows. Jack's finished with classes; that's right, Jack is.
But one use that it is very often confused is "it's". In 100% of the use of "it's", it simply means "it is". If you can't say "it is", then it's not "it's", it's "its".
When you want to use the possessive form of it, that's when you use "its", as in, a skunk has a stripe on its tail. Even though this is a possessive form, "it's" is never possessive, it's only a contraction.
But I see it every day. People are committed to use "it's" as in the possessive meaning. I even saw "it's" used incorrectly in a movie today.
If you can't say "it is", then it's not it's, it's its.