One of the uses of the apostrophe is to show possession, which means who something belongs to.
the girl's ball = the ball belonging to the girl
If the ball belongs to more than one girl, we make the world into a plural by adding s, then place the apostrophe at the end of the word.
the girls' ball = the ball belonging to the girls
The word girl is a singular noun, but the word girls is a plural noun (plural means 'more than one').
Most plural nouns in English end in 's' so the apostrophe comes after the 's'.
Here is another example:
the boy's ball = the ball belonging to one boy
the boys' ball = the ball belonging to more than one boy
Let's practise using apostrophes to show possession with plural nouns.