TV jokes that confuse me
Scene 1:
[Monica got a new bed in the shape of a racecar, and she and Phoebe are trying to persuade chandler that it’s just a regular bed so he won’t get too excited]
Chandler: It's a racecar.
Phoebe: So. This has always been Monica's bed, what you're just noticing now, how self-involved are you?
Chandler: Okay, well it this bed isn't new, how come there is plastic on the mattress?
Monica: 【Sometimes I have bad dreams】. (starts to break down, and Phoebe offers her hand to comfort her.)

Scene2:
Mr. Kaplan: (entering) How's that coffee comin,' dear?
Rachel: Yeah, right away Mr. Kaplan.
Mr. Kaplan: I'm not supposed to drink coffee, it makes me 【gassy】.
Rachel: I know!

Scene3:
Joey: Since when do you roller blade?
Phoebe: Oh! I met this really cute guy in the park and he like y'know, jogs, and blades, and swims, and so y'know we made a deal that's he's going to teach me all sorts of 【jock】 stuff.
Ross: And what are you going to do for him?
Phoebe: I'm going to 【let him】.

Qs below...

Question

TV jokes that confuse me
Scene 1:
[Monica got a new bed in the shape of a racecar, and she and Phoebe are trying to persuade chandler that it’s just a regular bed so he won’t get too excited] 
Chandler: It's a racecar.
Phoebe: So. This has always been Monica's bed, what you're just noticing now, how self-involved are you?
Chandler: Okay, well it this bed isn't new, how come there is plastic on the mattress?
Monica: 【Sometimes I have bad dreams】. (starts to break down, and Phoebe offers her hand to comfort her.)

Scene2:
Mr. Kaplan: (entering) How's that coffee comin,' dear?
Rachel: Yeah, right away Mr. Kaplan.
Mr. Kaplan: I'm not supposed to drink coffee, it makes me 【gassy】.
Rachel: I know!

Scene3:
Joey:  Since when do you roller blade?
Phoebe: Oh!  I met this really cute guy in the park and he like y'know, jogs, and blades, and swims, and so y'know we made a deal that's he's going to teach me all sorts of 【jock】 stuff.
Ross: And what are you going to do for him?
Phoebe: I'm going to 【let him】.

Qs below...

Answer

all three are dirty jokes involving american slangs...

1: it insinuates that the plastic is to protect the mattress from being peed on when monica has bad dreams
2. gassy means inclined to fart here
3. jock means a kind of snobbish athletes / celebrities who love to take advantage of their fans, and "let him" here means to let him be a jock in front of her (shag her).

Answer

The context implies the 'bad dreams' likely lead to bed wetting, thus the plastic on the bed.

Gassy definitely refers to a farting issue that Rachel is well aware of, having experienced it.

I think the 'Jock stuff'  implies things that require athletic ability (in this case, the inferred nuance is expertise/stamina in the bedroom).  I say that because of how Phoebes character is portrayed on the show.

Answer

Q1 - not sure; maybe it's her saying something along the lines of... 'Sh... it's not going to work..'

Q2 - 'gassy' = the dictionary says: Slang Bombastic or boastful (http://www.thefreedictionary.com/gassy), or'inclined to be verbose' 'but my understanding would be 'flatulent' (https://en.oxforddictionaries.com/definition/gassy)

Q3 - 'jock' = . Informal. a person who enjoys or is good at sports; athlete. (http://www.thefreedictionary.com/jock)
'I'm going to let him' -> let him teach her what he said he was going to teach her

I hope this helps
Jo.

Answer

Friends is one of my favorite shows. If you ever have any questions about it in the future, message me! I've seen every episode.

Answer

questions are here:
Q1: why does Monica say she has bad dreams as an answer to chandler’s question about plastic on the mattress?
Q2： what does gassy mean here？the dictionary defines it as drinks containing too much gas. 
Q3: what is jock stuff？ And Phoebe is gonna let him what？

million thanks to those who help~~:)