http://www.poetryarchive.org/glossary/metre
“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””
Note on the word:iamb
How my heart sings 