The poem uses an extended metaphor, dialect, and imagery to convey the theme that life is difficult, but you still have to be persistent and a hard worker.
A metaphor is a comparison that is direct. It says something is something (or not something) as opposed to saying something is “like” something. For example, the poem begins with a metaphor.
Life for me ain't been no crystal stair.
It's had tacks in it,
And boards torn up,
And places with no carpet on the floor --
Life is compared to a crystal stair, or an easy and beautiful journey. The mother is telling her son that life has been difficult for her, and there have been barriers to her success.
Dialect is also used in the poem. Dialect is the special kind of speaking certain groups use. In this case, the dialect is likely that of a working class woman.
I'se been a-climbin' on,
And reachin' landin's,
And turnin' corners,
In this case, the words are written as the speaker would say them and not in standard English. This helps to characterize the speaker and tell us more about her, such as her upbringing and education level.
Finally, the poem has a lot of imagery. It is so descriptive that you can really see what the speaker is describing. The descriptions of tacks and splinters and the image of the crystal stair itself are very specific and allow you to visualize the difficulties.