HDMI switching is what you'll have to do when you own multiple HDMI devices (HD players f.i.), with only one HDMI port (an HDTV with only one HDMI connection f.i.) to plug them in. An HDMI switcher is simply a device, that let's you change the source (output of the HD players) of the HDMI port (input on the HDTV) so that you can plug multiple HDMI devices in on only one HDMI port.
You won't need an HDMI switcher when you don't have more than one HDMI device, or have enough HDMI ports to plug them all in.
I hope that explains.
