Apostrophes have two main uses.
They can show possession, which means who something belongs to.
For example:
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.
For example:
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.
Want a bit more help with this before you begin? Why not watch this short video?