In law or official texts, their refers to things or rights that belong to persons or groups according to rules or agreements.
The tenant must maintain their part of the lease.
The company respects their employees' privacy.
Their can talk about one person when you do not know if the person is a man or woman. This helps avoid saying 'his' or 'her'.
Someone left their phone here.
Each student has their own desk.
Their can describe qualities, actions, or features that belong to people or groups. It shows what they do, have, or look like.
The students raised their hands quickly.
The players showed their skills on field.
Their can work before a noun to show that something belongs to some people or things mentioned before. This is a very common use in simple English.
It is their car outside.
Their dog is very friendly.
