Why does William Wordsworth say "stop here, or gently pass" in "The Solitary Reaper"?

The line you're referring to occurs within the first few lines of Wordsworth's "The Solitary Reaper," and it helps to put it in context:

Behold her, single in the field, 

Yon solitary Highland Lass! 

Reaping and singing by herself; 

Stop here, or gently pass! (1-4)

At first glance, that fourth line is a little strange; who is Wordsworth talking to, and what is he trying to say? What's interesting here is that, depending on how you interpret the scene, Wordsworth is actually talking to the reader, and he's basically telling the reader to avoid disturbing the solitary reaper. By saying "Stop here, or gently pass," Wordsworth is telling us to appreciate the scene, or move along quietly. By addressing the reader more or less directly, he puts us into the poem in a very intimate fashion. As such, we get the sense that we're actually walking around the highlands with Wordsworth and witnessing the solitary reaper alongside him. Line 4 is a a deft technique that makes us experience Wordsworth's poetic vision in a vivid fashion.