A Wireless HDMI Video Transmitter sends 1080p video to a small receiver attached to your TV from a diatance. You will need one if your sources of video are in a different room than your TV, and / or you have awall-mounted Tv and you have to power TV and HDMI.
There are two main standards for transmitting HDMI signals: WHDI (5 GHz frequency, regular quality), and WirelessHD (60 GHz frequency, for HD video).
Keep in mind that wireless transmission can take some seconds to sync up (connect) and can also potentially/occasionally drop out.
The best wireless HDMI transmitter: Logear GW3DHDKIT Wireless HD Kit (reliable, 100 feet transmitting power)
The second best Wireless HDMI Video Transmitter: DVDO Air3C 60GHz (best use of 60GHz technology)
The best wireless HDMI transmitter upgrade: Logear GWHDMS52 Wireless 5x2 HD Transmitter (four HDMI and one analog component input, pricey)