The word they’re is a contraction for the words they are. An apostrophe takes the place of the letter a.
Bob and Mary announced they’re getting married. Bob and Mary announced they are getting married.
My parents told me they are at the store. My parents told me they’re at the store.
The words their and there sound the same as they’re. Sometimes that can be confusing.
Just remember that they’re really means they are.
Use the word they’re as a contraction for the words they are.