What does the last line of the story mean/symbolize?
In the last line of the story, the narrator hopes that her daughter, Emily, whom she fears has suffered as a result of her weak parenting skills, will know "that she is more than this dress on the ironing board, helpless before the iron."
In other words, she hopes that Emily will have an easier life than she had had. She hopes that Emily will become more than a conformist, that she will realize that it is she who shapes her life, not some outside, demanding and pressing force. She hopes that despite her less-than-perfect parenting and the harm she fears she has caused her daughter, Emily will discover the beauty that is within her and realize that she does not have to be what the world or her society tells her she must, no matter how much pressure is applied to her to force her to conform; Emily has the power within her to decide her own future and is not bound by her current or past circumstances--she is not "helpless before the iron," but rather, has the potential to shape her own life. The narrator only hopes that Emily sees that power within herself.
The last line of "I Stand Here Ironing" is a beautifully painful line. The narrator, who has been working away the entire time she's been talking to the listener, has been beat on by life. She's been "ironed," if you will, pressed flat by all she has been through. Her life is very hard and very sad, and what she expresses here is a desire for her daughter to know she's better than that, and that she can have a better life. She doesn't want her daughter to be "helpless before the iron"; she doesn't want her flattened and steamed by life.