A bus is a group of wires representing a bit vector. Rather than drawing and/or labeling the individual wires in a schematic, you can draw a single "bus wire" to represent all of the wires.
This visiwiki page does an adequate job of explaining the concept and the schematic notation.
https://vlsiwiki.soe.ucsc.edu/index.php ... _schematic
Adding a bus construct to PCB123 is on the roadmap, to be sure, but as of today (6/6/2016) has not yet been put on the developer's schedule.