An iambic foot (iamb) has a short syllable followed by a long syllable (SL or U/).
Trimeter is three feet per line.
Again in this example, there’s some deviation from the iambic rhythm – for instance, in the second verse, “Slid from the kitchen shelf” starts with a trochee (LS, or /U). This change is quite possibly deliberate, to emphasise the noise of the pans hitting the kitchen floor.
There’s also an extra short syllable at the end of some lines.
However, this poem is mostly perfect iambic trimeter:
The whiskey on your breath
Could make a small boy dizzy;
But I hung on like death:
Such waltzing was not easy.
We romped until the pans
Slid from the kitchen shelf;
My mother’s countenance
Could not unfrown itself.
The hand that held my wrist
Was battered on one knuckle;
At every step you missed
My right ear scraped a buckle.
You beat time on my head
With a palm caked hard by dirt,
Then waltzed me off to bed
Still clinging to your shirt.
Theodore Roethke – My Papa’s Waltz
A closer look at the feet in this poem
U = short syllable; / = long syllable; | = division between feet