This song comes from the 1959 Broadway musical The Sound Of Music, which was made into a popular film in 1965.
In the play, Captain Von Trapp uses it to describe the way he feels about his love for Austria that is slowly dying away due to the Nazis and the Third Reich.
The song was written by the show's composers, Richard Rodgers and Oscar Hammerstein.
Viewers also had no idea that it's wasn't Plummer's voice on the song. The plan was to have him sing, but despite lessons he took before filming, his singing voice proved far from film quality (especially when juxtaposed with Julie Andrews) and a professional singer named Bill Lee was brought in to overdub his singing parts.
The lyrics are in the book: page 81