What does the Mensa International symbol stand for?

The M stands for Mensa, and the globe atop it symbolizes the world.

The M element is meant to appear as an oblique 3-D view of a "table" (mensa means "table" in Latin). The founders of Mensa intended that the group be a "round table" or open cooperative forum. The close spelling of the Latin Word mens(mind) suggested the association.

(There is some speculation that the significance of the m being below the world refers to Atlas, the titan of Greek mythology that holds the Earth on his shoulders, symbolizing that the world rests on the shoulders of Mensans, but there is no evidence to suggest that the organization intended this meaning.)