In IIDX games, players recreate the musical score of songs available to them through the use of a DJ simulating controller. The player must coordinate their hands in order to hit the correct keyboard buttons, spin the turntable, or a combination of both in correspondence to color coded notes that approach the judgement bar displayed on screen.
Source: Wikipedia, "Beatmania IIDX", available under the CC-BY-SA License.
In beatmania IIDX substream, there was a different songlist that could be linked to a Dance Dance Revolution machine for simultaneous play. If either the Dance Dance Revolution player or IIDX player made mistakes, the other game became more difficult.
Source: Wikipedia, "Beatmania IIDX Substream", available under the CC-BY-SA License.