Irregular meter describes a poem whose rhythm does not settle into a consistent metrical pattern from line to line. Instead of repeating the same arrangement of stressed and unstressed syllables, the poem varies its beat, which can make the speech sound more natural, unsettled, or flexible.
The term can also be used more loosely for an otherwise regular poem that includes occasional metrical variations, but the main idea is a meter that is not strictly uniform.
Example
The road bends north, then west, then back again;
my footsteps break the pattern of the rain.
One line runs quick, the next one drifts and slows,
as if the poem itself forgot its rows.