This is a poem about memory. In it, the speaker states that it has been five years since he first visited the area around Tintern Abbey. He states that he has changed in those five years. Back then, he was, as he describes it,

more like a man

Flying from something that he dreads, than one

Who sought the thing he loved.

How oft, in spirit, have I turned to thee,
O sylvan Wye!

Sylvan means wooded, and the Wye is a river. Memories of the beauties of this wooded setting with a river winding through it have often sustained him and made him happy. He will go on in the poem to speculate that the scene he is viewing will continue to provide both him and his sister Dorothy with a stock of joyous memories.

Returning to the same unchanged spot in nature offers two benefits to Wordsworth: it is way to measure his personal changes against a place that is the same, and it is a way to store up happy memories.