The Big Big Cheese is the fictional boss of the CBBC office on CBBC. He has never been seen or heard. He does, however phone in often (a lot more when Ed & Oucho were main presenters).
He has also talked to Ed on the phone in Ed and Oucho's Excellent Inventions. The name Big Big Cheese came probably because of the name Big Cheese which is what bosses are usually called, and that if you add another "Big" at the start, you have BBC in the first letter of each word.