The mother's advice to her son, as summarized in lines 15-20 of the poem "Mother to Son" by Langston Hughes, is to keep moving forward in life even though it may be difficult and that there will be obstacles to overcome. She tells him not to give up or turn back, even when the stairs become broken or hard to climb. She says: "So boy, don’t you turn back. Don’t you set down on the steps ’Cause you finds it’s kinder hard. Don’t you fall now— For I’se still goin’, honey, I’se still climbin’, And life for me ain’t been no crystal stair." The mother is using the metaphor of a staircase to convey the struggle of life and the importance of perseverance.
Therefore, the mother's advice is to keep going forward and persevere through life's difficulties, as shown in lines 15-20 of the poem. The evidence to support this can be found in the lines quoted above.