This time is is a Mini DisplayPort to HDMI Converter.
It sounds complicated, but it is just a cable which allows you to connect a HDMI-cabel from your Mac to your television.
And it is al plug and play. Not as Windows where they are all like 'Ooh, you've connected a tv to the computer, do you want to duplicate the screen?' Nothing like that at all. Just connect, and enjoy your huge screen in Full HD!
No worries. Just look it up in your manual, and you'll be fine.
Also made a quick video about the connecter. Go check it out!