The apostrophe signifies either omitted letters or possession.
We often omit sounds and letters when speaking, for the sake of convenience. The apostrophe comes in place of the dropped sound or letter. That explains the apostrophes in isn’t (is not), don’t(do not) and you’re(you are). The word apostrophe comes from the Greek word apostrophos which refers to a mark used in Greek to signify an omitted letter. It literally means a mark of turning away.
Possessive apostrophes help us understand relationship. Instead of saying the bag that belongs to my mom, we can add ‘s and say mom’s bag. If the singular possessor ends with an s, add an apostrophe after the s, as in Manas’ bag. In case of more than one possessor, the apostrophe is placed after the s, for example, girls’ changing room(changing room of many girls). But for plural nouns that don’t end in s, we add ‘s, as in women’s right or children’s park.
Last but not the least, for joint possession, the apostrophe is added to the word nearest the object of possession. Thus we would say Sindhu and Bindu’s parents.
That’s the apostrophe with its mighty uses.