Explanation of solitary reaper stanza wise?

In the first stanza the speaker comes across a beautiful girl working alone in the fields of Scotland (the Highland). She is "Reaping and singing by herself." He tells the reader not to interrupt her, and then mentions that the valley is full of song.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.

The second stanza is a list of things that cannot equal the beauty of the girl's singing:No Nightingale did ever chaunt

More welcome notes to weary bands

Of travellers in some shady haunt,

Among Arabian sands:

A voice so thrilling ne'er was heard

In spring-time from the Cuckoo-bird,

Breaking the silence of the seas

Among the farthest Hebrides.

In the third stanza the reader learns that the speaker cannot understand the Words being sung. He can only guess at what she might be singing about:Will no one tell me what she sings?--

Perhaps the plaintive numbers flow

For old, unhappy, far-off things,

And battles long ago:

Or is it some more humble lay,

Familiar matter of to-day?

Some natural sorrow, loss, or pain,

That has been, and may be again?

In the fourth and final stanza the speaker tells the reader that even though he did not know what she was singing about, the music stayed in his heart as he continued up the hill:Whate'er the theme, the Maiden sang

As if her song could have no ending;

I saw her singing at her work,

And o'er the sickle bending;--

I listened, motionless and still;

And, as I mounted up the hill

The music in my heart I bore,

Long after it was heard no more.