The lines that you are referencing come near the end of the rather long poem and serve to sum up the thematic intention of the whole poem. The poems is about his traveling to visit the ruined Tintern Abbey with his sister. In the poem, the speaker reflects on this past experience at this place five years earlier and how he "once against beholds these steep and lofty cliffs." He goes on to list all of the beautiful sights of the locale. He tells us that in those intervening years he has fondly remembered this place and the feelings that the place evoked and about the inspiration that these memories have had. He tells us that he stands here in the present that he has "pleasing thoughts [and] that in this moment there is life and food / for future years." All of these observations bring him to the moment of understanding that is conveyed in lines 88-93.
For I have learned
To look on nature, not as in the hour
Of thoughtless youth; but hearing oftentimes
The still, sad music of humanity,
Nor harsh nor grating, though of ample power
To chasten and subdue.
He is explaining that he has learned of the power of nature to have long lasting effects on the observer. In youth, the observer may be 'thoughtless' because he is only caught up in the moment. But the memory of the experience in nature can speak to the observer in surprising ways. Here he says that nature can both enliven and subdue with its multifaceted messages. These moments and inspirations help the observer of nature to actually better understand human nature. He becomes enlightened. As the poem continues after these lines, he elaborates his point and concludes this section of the poem with the thought that he will always be a "lover of the meadows and the woods" and all of nature because nature will always have a power over him.
In nature and the language of the sense
The anchor of my purest thoughts, the nurse,
The guide, the guardian of my heart, and soul
Of all my moral being.
This incredibly lyric poem is such a remarkable tribute to the Romantic idea of the power of nature on the human soul. Wordsworth discusses what seems to be his personal growth from youth to maturity in regards to his appreciation of the power of nature. As he speaks to his sister in the last section of the poem, he is speaking to his audience as well.