Those characteristics that correspond with the biological sex. More specifically, it’s about different categories of characteristics: chromosomes, internal sex glands, hormones, internal and external sex organs, and the further development during puberty.
The western binary model assumes two sexes: man and woman, both with well-defined and distinct sex characteristics. Those two groups are a part of the broad sex characteristics spectrum, but there’s a lot more bodily variation. People with sex characteristics that don’t fit in the traditional binary of man/woman have intersex bodies.