They are referring to the fact that drinking tea often stains your teeth and pretty easily. Black teas are heavy in tannins which do all of the staining.

I found that getting the fluoride paste applied at the end of each cleaning actually helps reduce the tea staining. (My teeth used to stain pretty bad)

My hygienist recommended the Crest 3D White toothpaste to help with my staining. I’ve been using it for about nine months now . I didn’t solve things completely but my teeth definitely look better and my last visit was at least ten minutes shorter.