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:

Who did?

But when "they" means "the people in charge", you don't need to explain who you mean.