Reproductive gender, often (confusingly) referred to as "sex", is the role which an individual plays in reproduction.
In the most common scenario, the female provides one ovum (or several ova, in some species), each of which are fertilized by no more than one spermatozoon. The male provides a much larger number of spermatozoa, which therefore effectively engage in a fitness competition to be the one which fertilizes the ovum. The female then hosts the fertilized ovum until it is ready to be delivered (in humans, as a baby).
While the word "gender" used to be more commonly understood to refer to "reproductive role" by default, usage is increasingly shifting towards referring to gender identity by default.