Biological sex is the division of biological species into male and female varieties, each also known as a sex. Only some species demonstrate sex.
Humans demonstrate biological sex and sexual reproduction, specifically a form of binary anisogamy in which females (women) produce low quantities of large gametes (ova or eggs), whilst males (men) produce large quantities of smaller gametes (sperm). This is typical of other mammals as well.
Biological sex is strongly correlated with social gender. A cis-gendered person has this correlation. Or it may not, in which case the individual is trans-gendered.