used to state what people in general say or think:
They say it's bad luck to spill salt.
3spokenused to refer to a particular organization or group of people:
Where are they going to build the new highway?
They're going to take an X-ray.
4used when talking about someone who may be male or female, to avoid saying 'he or she':
If anyone has any information related to the crime, will they please contact the police.
Every child, whoever they are, deserves to have a mum and a dad.
GRAMMAR GRAMMAR You can use they, them, and theirto refer to a single person when you do not want to show that the person is male or female. People do this because they want to avoid suggesting that the person can only be male, or using longer expressions such as 'he or she', 'him or her' etc• If anyone doesn't like it, they can leave. • When a friend upsets you, do you tell them? • Someone has left their coat.This use is acceptable and very common in speech, and is becoming more acceptable in writing as well.However, some people consider this use to be incorrect. You can sometimes avoid the problem by making the subject plural• If people don't like it, they can leave. • When friends upset you, do you tell them?
Definition from the Longman Dictionary of Contemporary English Advanced Learner's Dictionary.