The title informs us that the poem is describing an eagle. Otherwise, how would we know? As an interesting exercise, try showing the poem, without its title, to someone you know who hasn't read it. What did she think the poem was about? Did he guess that it was about a bird, or a bird of prey in particular?
But there's more. Chances are that your edition of the poem includes the title but not the word below it, "fragment," that appeared in editions from the 19th century. But that little word tells us that the poem we read today was just a part of a larger poem that Tennyson had imagined completing.
The poetic "fragment" was a big deal in the 19th century. The period of British Romanticism, which preceded the Victorian Age, had taught writers to think about incomprehensible things like infinity and the sublime. Artists had a hard time representing these huge ideas on limited canvasses or within the cover of a book. They would start a work with grand intentions to reveal the complexity of nature, only to realize that the task was too large for any artist to complete. The fragment was kind of the 19th century equivalent of saying, "Yada, yada…you know how the rest goes."