The words they are can be pushed together to become the contraction they’re.
They are coming to our party.
They’re coming to our party.
They are a cute couple.
They’re a cute couple.
A contraction always needs an apostrophe.
The word their is a possessive pronoun. The word their shows ownership.
The boys bought their father a gift.
My neighbors showed me their new car.
Do not use an apostrophe with the word their.
Use the contraction they’re when you really mean they are. Use an apostrophe in the word they’re. The word their is a possessive pronoun.
▶It’s your turn. Decide which word you need for the blank in this sentence – their or they’re? My sisters cleaned _____ room. Did you use the possessive pronoun without an apostrophe? Good for you.