In the Name of God, the Compassionate, the Merciful.
Hi!
A: Are these underwear too?
B: Yes, those are undershirts.
A: That you wear under a shirt.
What is “That” and what does it mean here?
Thank you
In this sentence, “That” is a relative pronoun.
It introduces the relative clause “that you wear under a shirt,” which provides additional information about “undershirts.” The clause describes the function of undershirts—specifically, that they are worn under a shirt.
Thank you so much, Torsten 
Very nice.
I think he’s leaving out words again. You could say:
“Something that you wear under a shirt.” Or maybe
“Shirts that you wear under a shirt.”
I would also suspect it’s actually question, with A: looking to confirm his understanding.
He’s leaving words out. It’s like A is finishing the sentence that B started. (Maybe A interrupted B?)
Yes, those are undershirts, that you wear under a shirt.
This makes it more obvious that it’s a restrictive relative pronoun.
A: Are these underwear too?
B: Yes, those are undershirts.
These and those are demonstrative pronouns. They refer to specific things. In this case they are physical objects that you can point your finger at.
I agree with Torsten. Let’s put A1 and A3 together with just a small difference, shall we?
By the way, Torsten, you speak a little Dutch, don’t you? In Dutch a relative clause is ‘een betrekkelijke bijzin’./bə.ˈtrɛ.kə.lə.kə bəi̯.ˈzɪn/
Thank you so much, Arinker
Very nice.
Thank you so much, NearlyNpping
Very nice.
Thank you so much, Masme
Very nice.
This clips sounds like a ‘Laurel & Hardy’ one. Not funny at all, because they usually gave me the jim jams, just like Harrold Lloyd. 
Especially the scene when he crawls up a building and hangs on clock tower. brrr. It begins at 48:12 minutes. Don’t watch the entire clip.
Hi Rezaforu,
It seems that you have developed a passion for underwear. Have you ever tried guick-dry whiner briefs or fast-drying-fuss pants?
I haven’t, but when you’ve tried them out, tell me about it so as to share your experience with me, would you?
@Torsten in Dutch these are referred to as ‘snelzeikerbroeken’. /ˈsnɛl.ˌzɛi̯.kər.ˌbrukən/
By the way, have you ever tried them MJTMN? After all, you’re the one who once wrote: ‘A person also has needs’. Perhaps these things might sexually arouse some women, because the last thing I know about your private indiscretions, as it were, was a woman who gave you a broad smile and walked away.
Keep trying old man.
Oh, and what happened to that girlfriend of yours, the one who had an alcohol problem?
Did you help her? No need to ask, really, because I’m sure you were not the cause of her problems.
Moreover, I can’t help being interested in your career. What would you want to be next, a surgeon, perhaps? I hope to live to see the day when that would happen.