Credit cubes were credits in a form of a small shiny cube that was about the size of a Human hand. They were similar to credit chips in that one cube could hold anywhere from a small to an extremely large amount of credits.
In 20 BBY, Supreme Chancellor Palpatine paid Boba Fett a large amount of credits with a credit cube for the information Fett had given him. It was information Palpatine already knew, but he paid Fett to keep the young man silent.