This is one of the most important ideas of "Tintern Abbey." The speaker of this poem has discovered, in his maturity, that his appreciation of natural beauty has allowed him to recognize a divine power in nature. Wordsworth comes up with this idea in "Tintern Abbey," and then really explores and develops it at length in his much (much) longer The Prelude. Nature means several things in the context of this poem (and in most of Wordsworth's poetry, actually): it can mean 1) physical nature (a.k.a. the Great Outdoors), or 2) it can mean the sense of unity or connection between everything (a.k.a. the "Force" in Star Wars), or 3) it can refer to a divine "presence" in Nature, like Mother Nature.
Young William's enjoyment of nature is purely physical and unreflective; he is unable to access the transcendental joy felt by his older counterpart because he has not allowed his "eye" to be "made quiet" (47).
Because reading poetry is always a thoughtful, reflective process, reading "Tintern Abbey" is actually a better way of accessing the transcendental understanding of nature, which the speaker attained only after years of calm reflection.