lucky penny
Scene:

Ross is giving a speech at a conference, all his friends are coming with him, now they have to leave but nobody is ready, he rushes everyone to get dressed so they won’t be late. Phoebe enters and gets dressed up, Ross is very happy.

[Ross: Wow, hello! You look great!
Phoebe: Thank you! I know, though.
Ross: You see this, this is a person who is ready to go. Phoebe you, oh, you are my star.
Phoebe: Ohh, well, you're my lucky penny.]

Questions:
Why did phoebe say “you’re my lucky penny”? Is “lucky penny” an everyday expression?

Question

lucky penny
Scene:

Ross is giving a speech at a conference, all his friends are coming with him, now they have to leave but nobody is ready, he rushes everyone to get dressed so they won’t be late. Phoebe enters and gets dressed up, Ross is very happy.

[Ross: Wow, hello! You look great!
Phoebe: Thank you! I know, though.
Ross: You see this, this is a person who is ready to go. Phoebe you, oh, you are my star.
Phoebe: Ohh, well, you're my lucky penny.]

Questions:
Why did phoebe say “you’re my lucky penny”? Is “lucky penny” an everyday expression?

Answer

It's not an everyday expression. I think it refers to this old saying:

See a penny, pick it up.
All day long, you'll have good luck.

I think that she's just saying that Ross is her good luck charm. It's just meant to be funny because Ross said that she was his "star" (meaning that she's awesome or amazing), so she just says that he is her "lucky penny", meaning that when she sees him she has good luck all day. I wouldn't get caught up in the meaning of this though as it's not a common expression, at least not to me.