People often use "they" to mean the people who are responsible for something. For example:
They should clean up the sidewalks here.
In this example, you don't know or care who exactly is supposed to do this; you just think that the people or department which takes care of the sidewalks should do it.
"They" can mean:
- the government
- the police
- the people who manage a building
- the people who run large companies
In other cases, when you use the word "they", you have to explain who you're talking about first. Imagine that someone says:
They came to the beach house.
If you didn't know who the speaker was talking about, you would ask:
But when "they" means "the people in charge", you don't need to explain who you mean.