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 for the drawers through cables. 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 is to manage, or oversee, the contractors doing the SIR work for 4 racks in the Spacelab. I did modify some storage drawers - drawers like the ones above that are used for storing small hardware. I also had to make sure the liners were designed and built.