What does "a melancholy strain" is the sixth line mean?

Reading Comprehension: Read the quoted parts carefully and answer the questions.Behold her, single in the field,
Yon solitary Highland lass!
Reaping and singing by herself;
Stop here, or gently pass!
Alone she cuts and binds the grain,
And sings a melancholy strain;
O listen! For the Vale profound
Is overflowing with the sound.

What does "a melancholy strain" is the sixth line mean?
【正确答案】：

"A MELANCHOLY STRAIN" REFERS TO A SAD TUNE.

【题目解析】："a melancholy strain"指的是凄凉的语调。