"An Ancient Gesture" by Edna St. Vincent Millay can be thought of as a metaphor.
A metaphor is a comparison between two unlike objects, saying that one thing "is" another. In the poem the narrator of the poem compares herself to Penelope, the wife of Odysseus in Greek mythology.
I thought, as I wiped my eyes on the corner of my apron:
Penelope did this too.
And more than once: you can't keep weaving all day
And undoing it all through the night;
Your arms get tired, and the back of your neck gets tight;
And along towards morning, when you think it will never be light,
And your husband has been gone, and you don't know where, for years.
Suddenly you burst into tears;
There is simply nothing else to do.
The narrator identifies with Penelope saying that she also feels that she does work all day, only to have it undone at night, getting tired and stressed out, being unable to sleep because she doesn't know where her husband is.
In the mythological story, Penelope is separated from her husband Odysseus, who is at sea. Thinking her husband lost, many princes ask her for her hand in marriage. She agrees, saying that she will marry when she is done making a tapestry. She weaves the tapestry all day, but at night, she undoes the work. She believes her husband Odysseus to be alive, and she never wavers in her faithfulness to him.
In the second stanza the narrator compares her gesture of wiping away her tears to all real, "authentic" gestures of grief.
And I thought, as I wiped my eyes on the corner of my apron:
This is an ancient gesture, authentic, antique,
In the very best tradition, classic, Greek;
And in the last line, the narrator finishes with her ongoing comparison of herself to Penelope.
Penelope, who really cried.
In this way she completes the metaphor of comparing herself to the Greek wife, Penelope... waiting faithfully every day for her husband, with tears in her eyes, only to be disappointed every night.