The line "A waste of breath the years behind" is in iambic tetrameter.
One meter consists of two syllables. So if we look at the line, we see there are 8 syllables: "A/waste/of/breath/the/years/be/hind." This means there are four meters. In a poem, this is called tetrameter.
A poem is called iambic when the syllables follow a pattern of stressed and unstressed syllables. The pattern is stressed, unstressed, stressed, unstressed, etc...
