It is a category belonging to gender identity. Agender people neither identify as a man nor a woman. If a cisgender person is the one who identifies with the gender assigned at birth, and a transgender person is the one who does not identify with the gender assigned at birth, an agender person does not identify with either of the two. Neither with the one assigned nor with the other. When it does so partially, it is called demigender (demi-girl, demi-boy, etc.)
All these categories have a common element that is often easily assimilated when it is quite complex, and that is to identify with a gender. It is possible that, for most people, it is an unconscious process, but when we talk about agender people, we see that it is not a simple process.
People identify, or not, with what we call gender. And this complicates the issue a bit more. Gender, the concept of man, the concept of woman, the masculine, and the feminine, are nothing other than social constructions. What is typical of men and women today was not so in the past, nor will it be in the future. What was not typical of men in the USA was normal in Papua New Guinea. The gender changes with time and space.
So people, when we are born, are assigned something relative, a gender, as if it were really absolute, immovable, and imperishable. If we don’t dislike it, we stay as we are. If we dislike it, we change. But What if we don’t like any of them?
For a while, we will do whatever we want, behave as we please, and dress as we like, but from a moment on, we will suffer a constant confrontation with what our gender is supposed to be and what it is not supposed to be. That is gender surveillance. Our environment will say, this is a girl and this is a boy, and agender people will not see themselves either as one or the other. And it may be that way forever, or it may change over time.
Read also: The sexual behavior of the Sambia.
See also: Purple Unicorn, youtube channel by Nick.Field: Gender Identity