It describes the author's childhood in the Lake District and expands on the themes of imagination and melancholy developed in Lyrical Ballads.  
The full version of this work was published by Selincourt in 1926.  It was originally intended as the introduction to The Brook, an immense poem meant to deal with all aspects of human nature, and the poet's reason for writing it is given in his preface to The Excursion.   Written over a period of 40 years, it is subtitled "the growth of a Poet's mind."  It describes the author's childhood in the Lake District and expands on the themes of imagination and melancholy developed in Lyrical Ballads.  FTP, name this posthumously published autobiographical poem, the magnum opus of William Wordsworth.
The full version of this work was published by Selincourt in 1926.  
The Prelude