Secondary sexual characteristics are the ones that develop at puberty. It is not the anatomy someone was born with.
In males, this refers to the maturing of the testicles (and associated things) to be able to produce sperm, the development of facial, body and pubic hair, deepening of the voice and a change in musculature and body shape associated with puberty.
In females, this refers to the maturing of the female reproductive system leading to menarche and the subsequent menstrual cycle, development of breasts (together with nipples and mammary glands), growth of body and pubic hair and development of the feminine body shape.