When Love with unconfinèd wings Hovers within my gates, (1-2)
Love's wings are "unconfinèd," which suggests that there is some connection between powerful feelings (like love) and freedom. Perhaps freedom is simply a matter of loving somebody enough, or experiencing a similarly intense emotion.
When I lie tangled in her hair And fetter'd to her eye, (5-6)
"Tangled" and "fettered" are words of confinement. Here, though, the speaker imagines a type of confinement that is different from being in prison. This is his way of making the best of his situation, of imaging a type of imprisonment that is pleasurable.
The birds that wanton in the air Know no such liberty (7-8)
Birds seem really "free." We mean, they can fly, after all. Apparently, freedom is not just about being able to fly around in the sky, though, because the speaker claims he knows a type of freedom that trumps that ability.