From the Associated Press:
The undated diagram that was given to the AP by officials of a country critical of Iran's atomic program allegedly calculating the explosive force of a nuclear — a key step in developing such arms. The diagram shows a bell curve and has variables of time in micro-seconds and power and energy, both in kilotons — the traditional measurement of the energy output, and hence the destructive power of nuclear weapons. The curve peaks at just above 50 kilotons at around 2 microseconds, reflecting the full force of the weapon being modeled. The Farsi writing at the bottom translates "changes in output and in energy released as a function of time through power pulse."
As a point of comparison, "Little Boy" — the atomic bomb that was dropped on Hiroshima during World War II — had a force of about 15 kilotons.
UPDATE: Glenn Greenwald of The Guardian has presented what he calls "new, overwhelming, very compelling scientific evidence" that the graph is a "farce." The AP maintains it is "[continuing] to report this story."