The narrator describes this little girl as eight years old, with thick, curly hair. She has "fair" eyes, which means light-colored eyes, probably blue or light green. The narrator then says they were "very fair," which means, in this context, very pretty. The girl is also called "a little cottage Girl": this means she comes from the poorer classes who lived in the cottages that dotted the Lake District where Wordsworth lived and wrote many of his poems. The girl has a "rustic air" and is "wildly clad," which conveys the idea that she spends a good deal of time outside in nature. The narrator says the girl's beauty makes him happy.
This poem is typically Romantic in its celebration of the common person and its celebration of the child. The poem also celebrates nature, which was much beloved by the Romantics, in its approval of the many wild and rustic aspects of this lovely little girl.