They make words possessive. How to make a word possessive depends on whether the word is singular or plural. Sometimes people put apostrophes into words that end with s. For example, often writers add apostrophes to plural words that are not possessive. Just because a word ends with an s does not mean it needs an apostrophe.
Some words always cause apostrophe trouble.
This group includes his, hers, its, and theirs. That manual is theirs, not yours. It's, with the apostrophe, is a contraction. Whose, without an apostrophe, is a possessive pronoun. My friend John, who is mother is a doctor, is studying medicine.