Meter is a system poets and teachers use to categorize the rhythms of poetry.The rhythms of poetry are real, but the poetic meter, since it is an invention, is an artificial approximation of the predominant rhythm. As an analogy: to classify a particular animal, as a mammal is to categorize whales, mice, and all others. As scientists have found the category "mammal" to be a useful category to differentiate between mammals, insects, snakes and fish, so have poets found terms like iambic and trochaic useful differentiation's.
At the core of the meter, lies the basic building block of rhythm, the foot. An iamb is a foot composed of an unaccented syllable followed by an accented syllable: ba-BUM. Iambic pentameter (pent = five) is a line composed of five iambs: ba-BUM ba-BUM ba-BUM ba-BUM ba-BUM.
Here are two lines of iambic pentameter, by the poet Christopher Marlowe:
"Was this the face that launched a thousand ships
And burnt the topless tow'rs of Ilium?"
A scansion of these lines would look something like this.
Was THIS / the FACE /that LAUNCHED / a THOU- /sand SHIPS
And BURNT / the TOP- / less TOW'RS / of IL- / li-UM?
It is important to note that, although many poems are written in iambic pentameter, very few poems carry the meter as regularly as these two lines. Poets deviate from the predominant meter often. Not to do so would push all poetry into a crushing state of boredom. For example, here is Marlowe's next line:
"Sweet Helen, make me immortal with a kiss!"
Which could be scanned:
SWEET / HEL- en, /MAKE me / im-MOR- / tal with / a KISS!
In this scansion, only two of the of the feet are obviously iambic, and there is the problem of the additional half foot. Note that despite the variation of this line, taken in context with the preceding two lines, Marlowe's poem is most assuredly iambic pentameter.
The other tricky part to meter is not all accents are equal. In the "Sweet Helen" line, if we assign values to the way we might naturally speak the line (1 = heavy, 4 = light), we might get something like this:
Sweet / Hel- en, /make me / im-mor- / tal with / a kiss!
1 / 2 , 4 / 1, 3 / 3 2 / 4 3 / 3 1
In natural speech, you can give more stress to the unaccented syllable of some feet than you give to the accented syllable of others. Iambic is said to be the natural rhythm of the English language, but there are three other basic poetic feet, besides the iamb: anapest ( two unaccented followed by one accented syllable), trochee (opposite of the iamb: BUM-ba), dactyl (one accented syllable, followed by two unaccented, and counter point to the anapest). Besides pentameter, other lines are named for the number of feet: monometer (one foot), dimeter (two feet), trimeter (three feet), tetrameter (four feet), hexameter (six feet), heptameter (seven feet), octometer (eight feet), and nonometer (nine feet).
A line of three anapests would be anapestic trimeter.
"I am monarch of all I survey!"
I am MON- / arch of ALL / I sur-VEY
To scan the final stanza of the poem, "Song to The Men of England ," which is perhaps one of the easiest to reveal its meter:
With PLOW / and SPADE, / and HOE / and LOOM,
TRACE / your GRAVE, / and BUILD / your TOMB(-),
And WEAVE / your WIND- / ing-SHEET, / till FAIR
England be your sepulcher.
Similar to the Marlowe lines above, there is a somewhat problematic second line, missing the first unaccented syllable in the first foot. Trying it as trochaic, it is less accurate:
TRACE your / GRAVE, and / BUILD your / TOMB (-),
Using this technique, you can analyze the meter in the rest of the stanzas. The poem has a number of irregularities, of course, from iambic tetrameter, but understanding of the poem can be enhanced by looking for lines where Shelley deviates and why he does.
To remind you, meter is the basic rhythm that dominates the poem, but it is never so rigid that it forces the poem to step along in lockstep like an army marching. Poetry is like music or like dance; the rhythm sets the mood and gives cohesiveness to the work, but the joy and the artistry come in the variations.