their S1 W1 [possessive form of 'they']
belonging to or connected with people or things that have already been mentioned:
They washed their faces and went to bed.
The twins spend all their time together.
People had moved back into their own homes.
used when talking about someone who may be male or female, to avoid saying 'his or her':
Everyone is free to express their own opinion.
Each student will have their own course-work folder.