xrandr wants to know which output port you want to apply the new modeline to. Most modern graphics hardware supports more than one output port, for example an HDMI port plus a VGA port on a graphics hard or an internal LCD screen plus an external port on a laptop.
xrandr without any arguments to see a list of available outputs. The names of the outputs are usually of the form
LVDS1. This will also tell you whether or not each output is connected and what its resolution and available modelines currently are.