The maiden's song has a profound effect upon the speaker. So much so, that it remains in his heart long after he last heard it. What's particularly remarkable about the song's powerful effect is that the speaker doesn't even know what the words of the song mean, or what it's about. This "solitary Highland lass" is most probably singing in Scots Gaelic, which would account for the speaker's not being able to understand her.
But although the words of the song may be incomprehensible, the sad, plaintive tone of the music is not. It speaks directly to the weary poet, providing him with spiritual refreshment. Even though the precise meaning of the song may never be known to him, it still connects with the speaker's soul as it partakes of what Wordsworth describes elsewhere as "the still, sad music of humanity."
In listening to the solitary reaper's song, the speaker, in true Romantic fashion, is immediately struck by the primal unity of everyone and everything that exists on this earth; an almost musical unity that transcends artificial distinctions of language, enabling us to communicate with one another.