Lines Composed a Few Miles above Tintern Abbey, on Revisiting the Banks of the Wye during a Tour, July 13, 1798
The poem opens with the poet visiting a place called Tintern Abbey on the banks of the River Wye in southeast Wales. He's visited it before, but not for five years. He remembers almost every detail: the sound of the "mountain-springs," "this dark sycamore," and the "hedge-rows."
He looks back on the past five years that have gone by since his first visit to the place, and remembers how much the memory of this scene meant to him when he was cooped up in the city. In fact, he practically relied on his memories of the beauty of the place to keep him sane while he was living in "the din/ Of towns and cities" (25-6).
Now that he's finally back in the same spot again, he finds himself looking out at the landscape and experiencing an odd combination of his present impressions, the memory of what he felt before, and the thought of how he'll look back on this moment in the future. He imagines that he'll change as time goes by from what he was during his first visit: a kid with a whole lot of energy to "boun[d] o'er the mountains" (68). Back in the day, nature meant everything to him.
Now, though, he's learned how to look at nature with a broader perspective on life. He doesn't just look and say, "Holy cow, the view from up here is pretty awesome!" and then run "bound[ing] o'er the mountains" again. In other words, he used to enjoy nature, but he didn't fully understand it. Now he looks and is able to sense a deeper, wider meaning to the beauty in nature. He sees that everything in nature is interconnected.
It turns out Wordsworth's sister is with him during his present tour of the area, and he says that she still looks at nature in the same way that he did when he was a kid. He imagines how his sister will go through the same development and transformation that he did. One day she'll be able to look out at nature and imagine the interconnectedness of things, too. Then he imagines her coming back to the same spot years in the future, after he's dead, and remembering the time she came here with her brother.