The apostrophe, sometimes called an inverted comma has two main uses.
The apostrophe indicates possession or ownership.
For example: The girl's hat was green, (girl is in the singular).
This shows the reader that the hat belongs to the girl.
The girls' hats were green, (girls in this instance are plural, i.e. more than one girl, more than one hat).
This indicates that the hats belong to the girls.