That can be used as an adjective and a pronoun. It is the singular form of those.
Compare:
- Look at that man over there.
- Look at those men over there.
As an adjective that can refer to either people or things.
- This photograph is much better than that photograph.
- That boy says that he is tired.
When that is used as a pronoun without a noun, it can only refer to things.
Compare:
- Look at that photograph. OR Look at that.
- Ask that man to come here. BUT NOT Ask that to come here.
However, that can be used as a pronoun when we are identifying people.
- Who’s that?
- Is that Ruth?
In a colloquial style, that can be used as an adverb.
- I couldn’t walk that far. (= I couldn’t walk as far as that.)
Relative pronoun
That can be used as a relative pronoun. As a relative pronoun, that can refer to both people and things.
- The letter that came this morning is from my brother.
- Give help to anyone that needs it.
That as a conjunction
That can also be used as a conjunction.
- His manners are so bad that nobody invites him to a party.
- Bring it to the light so that I can see it better.
- You may borrow my car on condition that you bring it before 10 pm.
That can also be used in exclamations.
- Oh, that I could see you again! (= How I wish it were possible for us to meet again!)