There are a few themes in "I Wandered Lonely as a Cloud," also known as "The Daffodils." One theme is the imagination as it relates to poetry. For Wordsworth, poetry was "the spontaneous overflow of emotion reflected in tranquility." In this poem, the speaker experiences a connection with nature. He doesn't merely see pleasant landscapes; he sees life. This is evident in the lines where he uses personification. "Ten thousand saw I at a glance, / Tossing their heads in sprightly dance." (11-12)
Later, when he reflects upon this experience, he has a deeper understanding because he is tapping into a creative process (the imagination) in which he recreates (or makes himself open to) the feeling of that experience through his memory. This memory-recreation is the imaginative process in the mind and it is the process of writing poetry. For Wordsworth, the two are linked.
Just as this poem is about recalling an experience, or reflecting on emotion in a tranquil state, it is also about recalling the way we look at the world as children, with a more innocent sense of wonder. When Wordsworth writes "I wandered lonely as a could," the implication is that he is floating along aimlessly looking for any experience, anything new to gaze upon. This is like the child wandering, asking questions about everything because he/she is curious. As an adult, the speaker, although it sounds paradoxical, purposefully wanders aimlessly in order to try to experience nature in the same way and again to reflect upon it later.
These are two themes: the poetic/imaginative process and recalling the wonder of a child's perception of the world.