I absolutely adore this song. I think she really put all the emotions and built up feelings into this song. Glee Cast did an amazing cover to this song, but I feel that they just can't get the emotion and the same feel as the orginal.
To me, this song talks about how she loves this person but can't, or is afraid, to let them go for whatever reason is forcing them to part. I think you can really tell that this relationship has been alive for many years and is now ending so abruptly. There's just something so sad about this song, yet something so beautiful and joyous. I think that's why I love it so much :) it's very rare to feel those two emtions in a song.
Overall, just simply beautiful.