An HDMI Switch is the opposite to a Splitter, a Switch will allow multiple source devices to be sent to a single receiving device. Once this is done you can simply select the source you want to use, it is very useful to get a device with a remote control as well so you can easily switch sources with the click of a button. There are Switches which allow you to connect 3 and 4 HDMI source devices (eg: BluRay player, Set Top Box, Playstation 3, etc) to an input on your display device (eg. TV, Projector etc.).
For information on the difference between Switches and Splitters, see our blog post "Splitters & Switches - What's The Difference?"