Apollo 12 was part of the Apollo program at NASA. It was the sixth manned flight in the Apollo program, and the second one to land on the moon. It went to the moon in 1969. Charles Conrad and Allan L. Bean walked on the moon while Richard F. Gordon stayed in the shuttle orbiting the moon.
It was a goal of the mission to land more precisely. There was a little bit of a rivalry between the prime crew, who were all from the Navy and the back-up crew, who were all from the Air Force. That rivalry resulted in a number of pranks back and forth between the two groups.
It was the back-up crew that played a joke on the prime crew, altering their lunar checklist slightly. What the back-up crew did was to add nude pictures of playboy models to the list. Each lunar checklist was attached to the wrist of their suits, and each page had its own small playboy cover page.
It was probably a shock for the two astronauts Conrad and Bean, and it just goes to show that your job is never too serious to have a little fun.