The author, Richard Lovelace, was imprisoned for political reasons, and this experience undoubtedly served as the inspiration for his poem "To Althea, From Prison."
The overarching theme of the poem is that even when one is locked behind bars without physical freedom, it is possible to experience freedom in other ways. Each of the four stanzas examines a different experience of this freedom.
Stanza one says that when the speaker thinks of his beloved Althea and remembers his time with her, he is freer than the birds in the sky who have the freedom to travel anywhere.
When I lie tangled in her hair,And fettered to her eye,The Gods that wanton in the Air,Know no such Liberty.
Stone Walls do not a Prison make,
Nor Iron bars a Cage;