[Idiom] What does "That,s what it is" mean?

[Bamdad]

What does "That,s what it is" mean?

 

[birdeen's call]

You probably meant, "That's what it is."

Note that I've used the apostrophe, not the comma.

"That's" is the contracted form of "that is". So the sentence without contractions would be,

That is what it is.

Do you understand it better now? If not, please read the following.

It's difficult to explain meanings of sentences and phrases without any context, Bamdad. Why don't you tell us who said this sentence and what was said before and after that?

 

[Bamdad]

Well, My Teacher said that expression,
So, he usually says that after a explanation about somethings.

 

[Raymott]

Well, My Teacher said that expression,
So, he usually says that after a explanation about somethings.

Given that everything is what it is, the expression seems to be a tautology. However, the expression is used to claim that one thing is another thing.
Example:
A: Please give me some lunch.
<B gives A a slice of bread and a dry lettuce leaf.>
A: Hey, I asked for lunch!
B: That's what it is.
(Lunch is what it is. Lunch is what the slice of bread and a dry lettuce leaf are.) It is your lunch; that's all you're getting.

In your teacher's case, he probably means something like "That (the explanation he just gave) is the correct explanation of the problem.

You want an answer? Read above. That's what it is. (An answer is what the above is)