Joey has dated a girl.
In the apartment, Rachel asked Joey how was the date going, Joey said pretty good. Joey was going to work and Erin was in the bathroom. Joey did not want to have a serious relationship with Erin, so he asked Rachel to help him telling the girl about this. Of course, Rachel did not want to do that. Joey suggested Rachel do not tell her directly, but from other side to let her know they will not be together. Like making some pancakes for Erin and starting a conversion to make it sound casually. Joey even mentioned that Chandler used to do it. Finally, Joey went to work and left Erin.
Erin came out from the bathroom, and asked Rachel if Joey said something about her.
Rachel obviously felt hard to say, said : Would you like some pancakes? (laughing)
I am trying to make sure what is the laugh point here ?
Is it because that Rachel was just trying to say something else, or she actually was doing what Joey has suggested to do: make pancakes, started a casually conversion, and then mentioned that Joey was not a good man? Or neither, there are some other stories behind it.
Yes, finally, Rachel did not tell Erin What Joey asked her to. When Joey returned home, Erin was still there, talking with Rachel.
Interpreting the meaning of something that someone says often requires seeing the facial expression and body language of the speaker.
I did not see this episode, so I did not see Rachel's facial expression or body language.
But maybe Rachel was shy to tell Erin what Joey had asked Rachel to say to Erin.
Sometimes when people are embarassed or shy to talk about a personal matter they laugh to show that it was not a serious matter.
This is especially true when you are asked (when Rachel was asked by Joey) to say something that soemone else (Joey) should say.
I hope you understand my 'best guess' answer.
