The meter of this poem is mostly iambic trimeter. That means there are supposed to be three (tri-) iambs per line. (An iamb is an unstressed syllable followed by a stressed syllable, which sounds like: daDUM. If you want to hear one in action, say the word "allow" out loud.) Now, we say "supposed to be" because most of the lines in this poem only have two iambs. Wait, really? Yes really. Every line in this poem, with the exception of lines 2 and 8, contains one anapest, followed by two iambs.
So what's an anapest, then? An anapest is a beat type that contains two unstressed syllables followed by a stressed syllable, as in line 4:
And the moon be still as bright.
So, if all the lines in this poem, with two exceptions, contain one anapest and two iambs, why call it iambic trimeter? Well even though there are anapests all over, there are more iambs. The iamb is clearly the dominant beat type in this poem. It's perfectly okay to say you're writing iambic trimeter and then to toss in some other types of beats, just for variety. And just in case you were wondering, the anapest sounds kind of like a little gallop or something like that. Their frequency in this poem makes it sound swift, fleet, almost as if it were marching to some particular destination, almost as if it were, dare we say it, roving, just a little bit? Are you with us? Right?
So, that's the meter. What about other aspects of structure? Usually, the poem is printed in three stanzas of four lines each (these are called quatrains). The poem also rhymes, with a rhyme scheme that looks like this: ABAB CDCD AEAE (were each letter represents the last, rhyming sound in that line). Notice that the "A" rhyme returns in the poem's final stanza. This is pretty cool; the poem is coming full circle.
There's a lot of ways you can interpret the repetition of this rhyme, so we'll just give you one. The poem is all about roving, about wandering around with no particular destination. Well guess what? The rhymes in this poem have a destination. They start at one place, and return to it later. In other words, it doesn't roam all over the place; it keeps things nice and compact. Just as the speaker refuses to keep roving, the rhymes themselves come home to stay, too.
Here's the other little thing to notice about the poem's rhymes. The sequence of the "A" rhymes is this: "roving"-"loving"-"loving"-"roving." Do you see how the two occurrences of the word "loving" are sandwiched in between the two occurrences of the word "roving"? Well, that folks is called a chiasmus, which you can read about right here. The chiasmus complements the rhyme, or is part of it. It makes the poem seem neat and compact, balanced and purposeful—just like the speaker is painting himself to be.