This means that the selected tap or shower tray comes supplied with a suitable waste. For showers, this is normally a great value fast flow waste (the unit that goes in the plughole).
For basins, this will either be a push waste, or a pop-up waste. Push-button wastes open and close by pressing the plughole itself, whereas pop-up wastes are operated by a lever behind the tap. Pop-up wastes are more commonly seen in traditional-style taps.