Metre is from the Greek word for measuring; at its most basic, metre is a system of describing what we can measure about the audible features of a poem. The systems that have been used in history to structure metres are: the number of syllables (syllabic); the duration of syllables (quantitative); the number of stressed syllables, or accents (accentual); and combinations of the above. English is not a language that works easily in quantitative metre (although this has not stopped people trying), and it has developed an accentual-syllabic metre for its formal verse. This means that, in a formal poem, the poet will be counting the syllables, the stresses, and keeping them to a pattern.
To describe the pattern, the stressed and unstressed syllables are gathered into groups known as feet, and the number of feet to a line gives a name thus:
1 foot: monometer
2 feet: dimeter
3 feet: trimeter
4 feet: tetrameter
5 feet: pentameter
6 feet: hexameter
7 feet: heptameter
8 feet: octameter
Lines of less than 3 or more than 6 feet are rare in formal poems.
The pattern of the syllables within a foot is also noted. A foot that is one unstressed syllable followed by a stressed one, for example, is an iamb; three of these in a row would be an iambic trimeter, while five make the famous iambic pentameter. All the common feet are outlined under 'Foot' in the glossary.
Like the rhythm in a piece of music, the metre is an underlying structure. Poets often slip in extra feet, or remove them, or change stress patterns around to prevent monotony, like playing rubato. (Sometimes a poem seems to be exploring how far a line can be pushed without losing all connection with the underlying metre.) This means that the discovery of a foot other than an iamb in the middle of what is otherwise iambic, say, does not stop the poem from being iambic; rather the attention ends up lingering at that point, so the word on the different foot ends up more powerful as it has the attention longer. An example of this can be found in Peter Dale's 'Half-Light'; he writes "I'm trying not to give another glance. / Lit window thirty years back up that path." The first line is a perfectly regular iambic pentameter, but the second introduces an extra stress on "Lit", so that what the speaker's trying not to be drawn to seems more powerful, perhaps helping us empathise with him when he does look back and "catch her eye an instant".