Practically everything. It's association with sex, its forms of expression, the way we identify it, the ways in which we personally relate to and understand it, the range of values various cultures assign to it, the relation of those values to who is expected to belong in which particular category, the biodimorphic qualities associated between sex and gender, the association and acceptability of masculinity/femininity within certain contexts and roles, the way we associate and express gender physically, verbally, through our work, through our values, through our perspectives. It's a aspect of human sociology that probably has the greatest and widest range of impacts on our day to day life and, more broadly, on our culture. It's ubiquitous, and equally complex.
Where would you like to start?