A towhead is someone who has very light blond hair. It's most common to describe blond children as towheads.
A true towhead has hair so light that it's nearly white, which is why most towheads are kids. Typically, blond hair grows darker as people age — even if you are blond as an adult, you hair will still be a darker shade of blond than it was when you were a young child. The word comes from the noun tow in the sense of "flax," which is usually a golden color.