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Jimmy: Your letter was a little Da Vinci Code
and l'm no cryptologist but l brought this
to remind you why we belong together.
lt's the cocktail napkin from that July 4th
party where you gave me your number.

Chloe: Oh, the Granville Carnival.
This was the best first date ever.

Jimmy: And my favorite. A piece of stained glass
from the bullpen that broke during that weird earthquake last fall.

Chloe: Okay, now l'm the one
that needs the decoder ring. She needs translation?
Jimmy: We lost touch for the longest time (the same as for so long?) and the day that we bumped
into each other again, the world... well, it literally shook.
And if that's not a sign, l don't know what is.
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About the "decoder ring" ... it's clear what she's referring to, if you have either read the book or seen the movie.
It's the tool that provides you with the key to understand a coded message, for instance by coupling each letter of the alphabet with another letter (or symbol, or number). When you write the message, you only need to replace the actual letters with the corresponding letters/symbols/numbers given by the ring. Without the ring (and without knowing how to use it, which should be the secret shared between the writer and the intended recipient of the message only) the message would be meaningless for the casual reader.

Here's an example I found on the Internet. (not the decoder ring from the movie, though).

Comments  
Chloe: Okay, now l'm the one
that needs the decoder ring. She needs translation?` It's not clear to her what Jimmy is trying to say.

Jimmy: We lost touch for the longest time (the same as for so long?) and the day that we bumped Yes. "for a long time" is also a good substitute.
Site Hint: Check out our list of pronunciation videos.
Thank you, Ray!
 Tanit's reply was promoted to an answer.
Thanks for the additional info, Sabs!
Students: We have free audio pronunciation exercises.