Apostrophes have two main uses.
They can show possession, which means who something belongs to.
the dog's dinner
Here, the apostrophe shows that the dinner belongs to the dog.
They can also be used for contraction, which means missing out letters from a word.
The dog doesn't want to eat its dinner.
Here the apostrophe shows that the letter 'o' has been missed out. Doesn't is short for does not.