An iambic foot (iamb) has a short syllable followed by a long syllable (SL or U/).
Tetrameter is four feet per line.
Note the change in metre in the last line of the following verse – it actually starts off long-short-short (LSS or /UU), which is a dactyl. This varies the rhythm, giving a pleasing effect.
I wandered lonely as a cloud
That floats on high o'er vales and hills,
When all at once I saw a crowd,
A host, of golden daffodils;
Beside the lake, beneath the trees,
Fluttering and dancing in the breeze.
William Wordsworth – from The Daffodils
A closer look at the feet in this poem
U = short syllable; / = long syllable; | = division between feet