As opposed to a stair that gives a passage to where we want to go, her life is unsure. It was unsure and full of the unknowns, yet she tries to move forward with all her strength.
The mother here is trying to teach his child a lesson about the uncertainties of life and how life can be so unfair sometimes.
This is a wonderful poem about how mother's try to survive. And that motherhood is not an easy path to take. Only those who are strong inside can carry on with the uncertain journey.