Hard-wiring means fitting a DCC decoder to an item of rolling stock without the benefit of a simple socket arrangement.
Hard-wiring means you need to do a small amount of soldering and cutting of wires.
Hard-wiring D&E models is normally a pretty simple operation. Most 4mm scale locos, for example, have plenty of space inside and contain simple wiring with no hidden pitfalls. The odd loco such as the Bachmann Class 04 can be a little more taxing, as can smaller N gauge models, but generally even these are not too difficult.