It was in this video. It is right at the beginning. Here it goes:
[Speaker 1] Mr. and Mrs. Mountbatten-Windsor, correct?
[Speaker 2] Well, to be.
Does speaker 2 mean yes we are?
Speaker (2) means "we will be" in the future. They are not yet 'Mr. and Mrs. Mountbatten-Windsor' because they are not yet married. The phrase 'to be' can mean 'in the near future'. A bride to be is a woman who will soon be married. A mother to be is a woman pregnant for the first time. These phrases can be hyphenated e.g. bride-to-be. The introductory 'well' is a polite way of introducing a correction.
"husband and wife to be" is an idiom that refers to a couple that are currently engaged to be married, but are not actually married yet.
When the first speaker says "Mr. and Mrs. Mountbatten-Windsor, correct?", this implies that they are already married. (In English, a married, mixed-gender couple who share a last name are sometimes referred to as "Mr. and Mrs. ____ ".)
So the response "Well, to be." could be re-written as "Well, we're not married yet, but we will be."