What does the white strip mean here?

White stripes: printed on paper showing white characters on a red background, and the characters on the printed surface are concave.

For example:

Seal cutting seals are divided into two types. One is Yang Wen, whose characters stand out on the printed surface and are red on the paper. Therefore, this Yang Wen seal is also called "Zhu Wen".

For example:

The other is yin, the characters are concave on the printing surface, and the printed paper is white. Therefore, this kind of seal is also called ""in Yang.

Seal cutting seals are mostly used in calligraphy works.