The theme of "Mother to Son" by Langston Hughes is the mother's urging of the son to keep going in life, despite difficulties. The theme is reinforced by the extended metaphor of the crystal staircase. Instead of climbing a crystal staircase, the mother climbs a staircase with "tacks," "splinters," "boards torn up," "and places with no carpet on the floor." All of these images emphasize the difficulty of the mother's journey upward and her need to push forward despite obstacles in her path. The diction the mother uses is vernacular speech--the speech of everyday. She speaks the way a woman who did not have the benefit of an education might speak--a woman who's always had to work hard for what she has. For example, she uses words like "ain't" and phrases like "no crystal stair." Other examples of her vernacular speech are "I'se" and "a-climbing." Her diction reinforces the idea that she comes from a humble background but keeps working to reach something better.
The whole theme of Langston Hughes's "Mother to Son" is that it is necessary to keep working hard, to keep climbing, in order to get ahead. The diction (choice of words) and imagery of the poem reinforce that.
The central image of the poem is the stairway. This image encourages us to think about the upward progress that the mother wants her son to make. This image, with its talk of splinters, and twists and darkness reinforces the theme that one should keep working upwards, no matter the obstacles.
The choice of words is, I believe, meant to illustrate how the mother has not herself "made it." She uses "dialect" much of the time and sounds uneducated. But the point is that she has worked hard and given her son a base and he needs to keep climbing from where she has gotten him.