In short, the 'gender binary' is the concept that there are only two genders - male and female - and that former is somehow superior to the latter. This was not born out of biology, as many believe, but rather a system of oppression and power. The gender binary, especially how we experience it today, is rooted in violent white colonization's fight for global power. Learn more at the following resources.
Beyond The Gender Binary | GLSEN
The Gender Binary Is a Tool of White Supremacy | by Kravitz M. | An Injustice! (aninjusticemag.com)
Pre-colonial communities’ history of gender fluidity - BBC News
Colonialism Still Affects How Black and Indigenous People See Gender | them.
Los Muxes: Disrupting The Colonial Gender Binary — Human Rights Pulse