It's made famous by the Shawshank Redemption as a symbol of hope and to quote one of the characters in the movie:

"I have no idea to this day what those two Italian ladies were singin' about. [bad word] is, I don't want to know. Some things are best left unsaid. I like to think they were singin' about something so beautiful it can't be expressed in words and makes your heart ache because of it. I tell you, those voices soared, higher and farther than anybody in a gray place dares to dream. It was like some beautiful bird flapped into our drab little cage and made those walls dissolve away. And for the briefest of moments, every last man at Shawshank felt free."

From the Marriage of Figaro: The aria takes place in Act III and Scene IX.

During the first part of the duet (bars 1–37), the Contessa dictates the title and the three lines of the letter and, after a pause, Susanna repeats the lines as she writes them. In the second part, the Contessa and Susanna read alternate lines with a slight overlap (bars 38–45) until they finish in a [bad word] duet with their conclusion.

On the breeze... What a gentle zephyr... Be this evening... Zephyr...

Be this evening... Be this evening...Under the pine grove.
Under the pines... Under the pine grove.
Beneath the grove of pines ... And will understand the rest.

Certainly, he will understand. Certainly, he will understand.

Tune on ...What a gentle zephyr ...Be this evening ...
Under the pine grove. And will understand the rest. Certainly, he will understand. Certainly, he will understand. Certainly, he will understand.

He'll understand ...He'll understand ...He'll understand ...
He'll understand ...He'll understand ...He'll understand ...

In Italian: The duet's time signature is 6/8, its key is B-flat major and it is 62 bars long; the tempo indication is allegretto. Scored for oboe, bassoon and strings; the duet has a vocal range from F4 to B♭5 for Susanna and from D4 to for the Contessa. 

Titile: Sul..l'a..ria.... 

[Part1 - First 3 Lines with Reiterations]
Che... so..a..ve.... zef..fi..ret..to.... (zef..fi..ret..to....)
Qu..e..sta.. se..ra... spi..re..rà.... 
Qu..e..sta.. se..ra... spi..re..rà....

Sot..to... i... pi..ni.... del... bos..chet..to.... (Sot..to... i... pi..ni....)
Sot..to... i... pi..ni.... del... bos..chet..to....
Sot..to... i... pi..ni.... del... bos..chet..to....

Ei... già... il... re..sto... ca..pi..rà....
Cer..to... cer..to... il... ca..pi..rà.... 
Cer..to... cer..to... il... ca..pi..rà....

[Part2 - Alternate Overlapping Suprana's]
(Can..zo..net..ta... sul..l'a..ria....) Che.... so..a..ve... zef..fi..ret..to.... 

(Qu..e..s..ta... se..ra... spi..re..rà....) Sot..to... i.. pi..ni.. del... bos..chet..to...
 
(Ei... già... il... re..sto... ca..pi..rà....) Cer..to... cer..to... il... ca..pi..rà.... 

(Cer..to... cer..to... il... ca..pi..rà....) Cer..to... cer..to... il... ca..pi..rà....

[Part3 - Toward [bad word] Duet]
(Il... ca..pi..rà....) Il... ca..pi..rà.... 
(Il... ca..pi..rà....) Il... ca..pi..rà....
(Il... ca..pi..rà....) Il... ca..pi..rà....