An iambic foot (iamb) has a short syllable followed by a long syllable (SL or U/).
Heptameter is seven feet per line.
I sang of leaves, of leaves of gold, and leaves of gold there grew:
Of wind I sang, a wind there came, and in the branches blew.
Beyond the sun, beyond the moon, the foam was on the sea,
And by the strand of Ilmarin there grew a golden tree.
Beneath the stars of Ever-eve in Eldamar it shone,
In Eldamar beside the walls of Elven Tirion.
There long the golden leaves have grown upon the branching years,
And here beyond the Sundering Seas now fall the Elven tears.
O Lórien! The winter comes, the bare and leafless day;
The leaves are falling in the stream, the river flows away.
O Lórien! Too long I’ve dwelt upon this Hither Shore
And in a fading crown have twined the golden Elanor.
But if of ships I now should sing, what ship would come to me,
What ship would bear me ever back across so wide a sea?
JRR Tolkein – Galadriel’s Song of Eldamar
A closer look at the feet in this poem
U = short syllable; / = long syllable; | = division between feet