'They' is a pronoun that is used to refer to a group of people or things.
It is a gender neutral pronoun that does not distinguish between male and female. Therefore, it is often used in place of 'he or she' to make the statement gender neutral. For example, 'We don’t know who they are'. It can also be used to refer to an indefinite group of entities such as 'They say we’re in for a cold winter'. When referring to a group, it is important to make sure the group is well defined so 'they' can be correctly interpreted.
a. They were talking about the upcoming election.
b. Will they be coming to the party?
c. They say it’s going to be a long winter.
When using 'they' to refer to a group, make sure that the group is well defined to avoid confusion.