Refer to lines 88-93 in Tintern Abbey by Wordsworth and explain the ideas of humanity, youth, experience and nature.

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. 

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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.