The poem is addressed by a prisoner to a woman named Althea. Not surprisingly, prisons, and the language associated with them, are everywhere in this poem. Descriptions that don't directly relate to being in prison are nevertheless laced with references to prisons or being restrained. Although the speaker claims that even though he's in prison, he's actually free, this type of language suggests that, perhaps, he really can't escape his prison.
Line 5: The speaker is "tangled" in Althea's hair. He's not literally tangled, so this is a metaphor for the way he is touching her hair, or the way in which he is captivated or enraptured by it.
Line 6: The speaker is also "fettered" with Althea's eyes. He's not literally chained to them, so again we've got a metaphor for the way in which he can't stop looking at them, the way he is lost or "imprisoned" by them.
Line 17: The speaker describes "committed [caged] linnets," which symbolize both imprisonment and song.
Line 25: Despite these references to prisons, the speaker claims that "stone walls do not a prison make."
Line 26: He tells us that "iron bars" do not make a cage either.