Did you google it ? (or other search engine of chocie)

First hit on google is

http://elinux.org/RPiconfig

if you search the page for that line it says

hdmi_force_hotplug Pretends HDMI hotplug signal is asserted so it appears a HDMI display is attached

 hdmi_force_hotplug=1 Use HDMI mode even if no HDMI monitor is detected