Percy Bysshe Shelley was a Romantic poet. The Romantics typically wrote about very similar things. The characteristics of the works typical of the Romantic poet are: love of nature, love of the uncivilized or natural, beauty in nature, the importance of imagination over reason (as a contrast to the Neoclassicists who preceded the Romantics), and individualism.
In Shelley's poem, "To a Skylark," Shelley wants the reader to understand the importance of the skylark. The skylark is a bird which brings beautiful music to the world with its song. Shelley understands the importance of the beauty of nature (animal life included) and the importance of mankind's realization that nature holds a special place in the world.
The comparisons made between the world of man and the world of nature show Shelley's ideology regarding the impact which nature should have on man and the respect that mankind should have, in return, for nature. In the end, Shelley's asking of the skylark to share with him the secrets of nature show that Shelley does, in fact, recognize the importance nature holds in the lives of mankind.