If you are asking what the word "THAT" means, I will try to explain one meaning.
The word "THAT" acts as an adjective or a pronoun. It means any THING (an item, object or person) at a distance.
If someone points to a chair across the room, they might say, "sit in THAT chair".
If someone asks, "what piece of cake do you want, you can point to one piece and say "that one".
The word "THAT" helps you to show the difference between one thing and another.
It can be used to show the difference between people.
For example, you could be asked, "which of those men is your brother?".
You can point and say, "THAT man, the one with the green shirt".
Hope this helps