An iambic foot (known as an iamb) has a short syllable followed by a long syllable (SL, or U/).
Monometer is one foot per line.
The bold text in this example indicates the LONG syllable – the one that is accented.
Robert Herrick – Upon His Departure Hence
The accent on a syllable can change even for the same word. For instance, the word “I” is long in one place, but short in another, depending on the way the poet has written the line.
A closer look at the feet in this poem
U = short syllable; / = long syllable