Consumer Electronics Control. It is supposed to help integration between equipment so when say you turn on your streaming box the TV turns on and switches to the correct input automatically.
Different companies can call it different things, Panasonic call it Viera Link.
When it comes to the X1 what happens is the Activity turns on the streaming box which using CEC turns on the TV then as part of the Activity the X1 tries to turn the TV on but this actually turns it off as its already on.