The rack contains mechanical parts, called a liner, used to slide drawers in and out, much like a file cabinet. Drawers are used to contain or repackage things and make them flight ready. For example, we can't take computers to space in the same outer packages or cases we use on earth because some pieces may float out or the vibration from launch would destroy them. On the rack side, the SIR also provides power and data connections for the drawers. Power for a drawer comes from a connector in the rack, just like a wall outlet at your house.
Data cables serve many purposes, but the main one is for communication.
For the computer drawer mentioned above, parts are wired to connectors in the drawer. When you slide the drawer into the rack, it connects to cables already in the rack. In general, my job was to manage, or oversee, the contractors doing the SIR work for 4 racks in the Spacelab. I did modify some storage drawers, the hardware engineer part of my work. These drawers are like the ones above but are used for storing small hardware in foam.