This should also be obvious, but I suppose not, judging by the state of things on the Internet.

They're is the contraction form of a subject and verb pairing. It takes they (the subject) and are (the verb), and then they become one entity. You use it subjectively. For example: They're so loud.

There is an adverb related to location. It's used when directing towards an area. For example: I left my coat over there.

Their is a possessive, so you use it when suggesting ownership. For example: Their cat clawed me last week.

Now you know.