"girls" is a singular form, which means a girl, while "Girls'" is a plural possessive form, which means all the relationships of multiple girls.

For example, "This girl is very smart." This girl is very clever. ) The singular form of "girl" is used here. And "these girls' backpacks are pink." A girl's schoolbag is pink. ) The plural possessive form of "girls" is used here, which means that many schoolbags owned by girls are pink.

So the difference between "girl" and "girl" is that they represent all the relationships between a girl and multiple girls respectively.