Lyrics is a fairly well known and unusual poem, unusual for its authorship: an accomplished Song Dynasty courtesan by the name of 聶勝瓊 (Nie Xin Qiong).
Legend has it that she fell for a client Li who transited through her city; within days of returning home, Li received Nie's poem composed for him; Li's wife was so taken by her writing that she persuaded him to take Nie as his concubine. Hence, within what is known (not very much) and just beyond the last verse of the poem, there was a happy ending, her sweet dream did come true. Indeed she talked of pursuing "good" dreams that were out of reach (寻好梦 梦难成); that dream undoubtedly would have been about being accepted by a good man and putting her life as a courtesan behind her.
It is remarkable for that era (undated writing, 10th to late12th Century): (1) women were rarely literate let alone capable of poetry, (2) for a woman to bare her feelings would have been unthinkable, (3) for a courtesan to approach a married client, especially of high ranks so openly was bold beyond imagination.
Similar to Geishas in Japan, she would have been raised as a courtesan in a 青楼 (literally: green mansion, most exclusive class of geisha houses), highly trained and exceptionally educated and likely became a 花魁 (huakui, reserved for the highest ranking/paying clients). Her particular establishment was called the Lotus Mansion and she hinted at that being a green mansion in the second verse (莲花楼下柳青青) by describing the green willows and the name of the establishment.
There have been many poems that share this title 鹧鸪天 (Zhe Gu Tian, literally: sky of partridges, I know nothing about the root of this term and am clearly far less educated than Nie). Somehow partridges were associated with farewells and separations; Nie's poem often carries an additional subtitle "Regards to Li" that distinguishes it from others.
Mention of 鳳城 (Feng Cheng, literally: Phoenix City) being the capital city offers a clue to the timing of this poem. This was the capital of Northern Song, Bianjing (modern day Kaifeng, 960-1127). There is modern day city 鳳城 in the far north, though that would have been well beyond the borders of Song.
There is much more to this song than just another one about sadness of separation.
It is about having "unworthy" feelings for someone "above her status", likely for someone she had not known for long that began as a transactional relationship. 尊前一唱阳关曲 (singing in front of dignitary) confirmed this.
* In the video, curiously the dignitary 尊 somehow became a bottle/flask 樽, though I am fairly sure that the original text had 尊.
Contextually, bottle is plausible given that wine would have been consumed and even as an attempt to contrast the hidden sadness against what would have been an openly joyous atmosphere.
An intended double meaning is also entirely feasible with deliberately ambiguous choosing of the character, especially with her mastery of subtleties throughout the poem. Also, in Cantonese (the song here), these characters have sufficiently distinctive pronunciations, yet in original Mandarin (I presume) and modern Putonghua, they share identical pronunciation.
I am more inclined to side with dignitary (or a double meaning) which depicts the power dynamics of the unequal relationship also hinted at elsewhere in the poem, all of which makes her dream more hopelessly out of reach.
It is unspoken whether he was oblivious to her affection; I feel that it was very much one-sided and kept hidden from him. They could not have been together for long - he was on a stopover and was about to continue on the fifth leg of his journey (别个人人第五程).
It is also fairly overt that she dared not share her feelings with anyone (有谁知我此时情), again difference in status (lest being mocked for dreaming beyond her station. Pretty Woman and La Traviata spring to mind). Whilst I translated that as "Who can know of my feelings at this moment", more literally, I am guessing that she did not want to say so directly that "I dare not share this moment's feelings with anyone". Given this, I am guessing that he was oblivious to her feelings or perhaps regarded that as part of the service.
The last verse also alluded to her impossible situation: she is tormented inside, yet she could never pursue happiness with this relationship or be open about her feelings (raining outside, she would have been tormented too but in other ways), for the division of their ranks/statuses (the window). Fulfilment, even an imagined one, seems hard to come by even in her dreams.