The odd-numbered lines of the poem are written in a meter called "iambic tetrameter," while the even-numbered lines are in "iambic trimeter." So what's all that about? This means that the odd-numbered lines contain four ("tetra-" is Greek for four) units of rhythm called "iambs." The even-numbered lines have three of these guys ("tri-" is Greek for three).
Great. So, what's an iamb? An iamb, as you have heard somewhere, is a beat (often called a foot) that contains an unstressed syllable followed by a stressed syllable. If you say "allow" out loud, you'll hear the basic rhythm of an iamb: da DUM. Let's scan the first line to see what iambic tetrameter looks like:
When Love with unconfined wings. (1)
(You might have noticed that "unconfinèd" is scanned as if it had four syllables; the little accent mark on the "ed" tells you that you should pronounce the word in this way: un-con-fin-ED).
For iambic trimeter, line 4 is a good example:
To whisper at my grates. (4)
Although poets like to change things up from time to time (it is rare for every single line of a poem to contain only iambs, for example), Lovelace is pretty regular in this poem. Nearly all of the poem's thirty-two lines match this rhythm very neatly.
Like lots of poems from the 1600s, "To Althea, from Prison" also has a regular rhyme scheme: ABABCDCD. This means that, in each stanza, the first and third lines, the second and fourth, the fifth and seventh, and the sixth and eighth all share an end rhyme. And the rhymes are pretty exact, too. Check out stanza two, for example, which rhymes "round" with "bound," "Thames" with "flames," "steep" with "deep," and "free" with "liberty."
Notice that each eight-line stanza is, essentially, subdivided into two quatrains (a group of four lines). The first half of each stanza, and the second half, are almost like their own little groups, with their own interlocked rhymes. So, what's the effect of such a regular rhythm and rhyme here?
Well, for a poem that's set in a prison, which is nothing if not regular and interlocking, the form and meter of the poem do a great job of representing a confined space. Nothing here is free or airy—nothing, that is, except for the speaker's imagination. The form of the poem, then, stands in nicely for the prison itself, while its content reminds us that that speaker's imagination remains free as bird… or a fish… or the wind… you get the idea.