Not sure if you are asking what the letters stand for, or what it is used for.
You now know what the letters stand for.
If you dont want to read the whole article then put simply, it is for connecting high definition devices together.
So for example, if you want to watch Sky in HD with a Sky HD box, you connect it to your TV with a HDMI cable.
If you want to play games in hgh definition you connect your PS3 to your TV with a HDMI cable.
I suppose you could think of it as the High Definition version of the old Scart cable.