From the memorial's web site:
At the entry portal, two stones are parted and a single stone wedge is pushed forward toward the horizon; the missing piece of what was once a single boulder. The smooth insides of the portal contrast the rough outer surfaces of the boulder. Beyond this portal, the stone appears to have been thrust into the plaza, wrested from the boulder and pushed forward – it bears signs of a great monolithic struggle.
On the visible side of the stone, the theme of hope is presented, with the text from King's famed 1963 speech cut sharply into the stone: "Out of the mountain of despair a stone of hope." On the other side are inscribed these words: "I was a drum major for justice, peace and righteousness”, a statement suggested by Dr. King himself when describing how he would like to be remembered.
The boulder is the Mountain of Despair, through which every visitor will enter, moving through the struggle as Dr. King did during his life, and then be released into the open freedom of the plaza. The solitary stone is the Stone of Hope, from which Dr. King’s image emerges, gazing over the Tidal Basin toward the horizon, seeing a future society of justice and equality for which he encouraged all citizens to strive.