The short form of "they are." This is the everyday spoken form; "they are" written out in speech usually means the speaker is stressing something (They ARE coming, whatever you think). It does everything "they are" does: describing — They're tired; locating — They're in the kitchen; and building the -ing tense — They're waiting for you. Like all uses of "they," it covers people, animals, and objects alike, and also the single person of unknown gender: Whoever left this, they're very kind. Register is friendly and normal — messages, speech, dialogue, most online writing. Strict formal writing tends to spell it out. Sound-wise it is usually one syllable, and here is the thing that trips everyone: in most accents it lands on exactly the same sound as "there" and "their." Your ear cannot separate them. Only the grammar of the sentence tells you which one it is.