The narrator of the poem is Wordsworth himself, and he is describing a scene that he supposedly saw while on his walking tour of Scotland. I say supposedly because Wordsworth did write the poem 2 years after completing his tour, and he also did explain that he got the idea from a sentence in a friend's book.
"This poem was suggested by a beautiful sentence in a Manuscript Tour in Scotland written by a friend, the last line being taken from it verbatim."
I guess not all poetry is inspired by beautiful scenery. Oh well.
The basic idea of this poem is that Wordsworth is walking along a path in Scotland. He sees a lone woman reaping in the fields. As she works, she sings. Her song is more lovely than just about anything else that Wordsworth has heard. It's more beautiful than birds singing (which for a Romantic poet is quite a thing). The third stanza is about Wordsworth wanting to know what the song is about. Is it old or new? Is it about far away battles or normal stuff? In the fourth stanza, Wordsworth says that the song topic doesn't really matter because it is so beautiful. How beautiful? So beautiful that he now carries the song in his heart even though he can no longer hear her voice.
If by "central idea" you mean major theme, I believe the poem points out the importance of seeing amazing beauty in normal, everyday, natural occurrences. The poem is about a woman working in a field, singing. I wouldn't think twice about it. But to Wordsworth, a Romantic poet, that scene is practically transcendent in what it has to teach mankind about the beauty of the natural and simple world.