"Femship" refers to the bond close female friends share. (Compare to "Menship") Friends is a broad term that can sometimes describe an acquaintance, a ride or die or anything in between. Femship indicates the depth of the relationship, and differentiates between a woman and another who is a casual friend and a woman and another who is her "sister from another mister".
It's an empowering term for women to publicly and proudly acknowledge their love and strong bond with the friend they consider "my girl" or "my sister" without being overly vulnerable. (We know vulnerability can be hard.)