I don't think that bus is an acronym or abbreviation but basically it refers to the main length of wire running beneath the baseboard that supplies current to the track. There will be two buses usually one red, the live, or right hand rail, and a black or neutral, attached to the left hand rail. Never the twain shall meet except by placing a locomotive or something else that you want to power across the tracks, or wired from them such as turnout motors, etc! The droppers refer to the smaller wires that attach from the main bus wires to each individual piece of rail. You would want to attach a dropper from each rail to the main bus because the rails do not conduct electricity all that well nor do the joiners between the rails leading to substantial drops in voltage and current over longer lengths, unless each rail is individually fed from the bus so that all the individual rails are effectively at the correct track voltage that is required for DCC running. Each of the buses will be a larger diameter wire on the droppers because even the buses them selves can have a voltage or current drop along their length if they are long particularly in large layouts.