Data Boards are pieces of hardware that the Lyoko Supercomputer requires to function. They are shown in the episodes Routine and Sabotage, each board is square is quite large in size, they are inserted into the supercomputer by opening various slots in the supercomputers outer ring. It is also revealed that Jeremie keeps copies of the data boards in his room just in case the supercomputer breaks down.
In Routine Jeremie had to take the boards out and swap them around as XANA had mixed them up to sabotage the Materialization program. In Sabotage, XANA possessed the Kadic gardener and used him to enter the Supercomputer room, remove the boards and damage them with electricity. The damage caused on the boards caused major glitches, bugs and power failures on Lyoko, Jeremie even quotes that some of the supercomputers functions were damaged as he couldn't launch a return in time.
Data boards are superconducting circuits that allow electrons to move with almost no resistance at low temperature which is why the supercomputer has a cooling system. The alloy used to make the circuit is unknown but its a glowing super conductor. The date boards are what allows the super computer to operate. As seen in "Sabotage" a damaged date board causes bug in the computers programing until the virtual world shuts down. As seen in "Frontier" rearanging the date boards rewires how the date is transferred an reprograms the computers date. The "Virtualization" and "Materialization" program.