What does it mean to "not be a valid string in Unicode"?

[Costello, Roger L.]

Hi Folks,

In the book, Fonts & Encodings (p. 61, first paragraph) it says:

    ... we select a substring that begins
    with a combining character, this new
    string will not be a valid string in
     Unicode.

What does it mean to not be a valid string in Unicode?

/Roger