It means women are 1/2 of the production of babies. Men are the other 1/2. It takes a male and a female. Therefore, they would like to be treated as equals when it comes to women's rights and financial matters. And maybe any other place they think they aren't being treated fairly.
I do not know if women are half the population or not. Maybe the women who made that statement are from countries where women are half the population.
They worded it wrong. They meant to say "women birth the entire population and we are half the population". I believe I read somewhere that women are over half the population