Kimberlé Williams Crenshaw first used the term in a 1989 essay called "Demarginalizing the Intersection of Race and Sex: A Black Feminist Critique of Antidiscrimination Doctrine, Feminist Theory and Antiracist Politics." She uses a traffic intersection as an analogy for identity: If an accident takes place there, you can't be sure which road it came from. In the same way, she writes, "if a Black woman is harmed because she is in an intersection, her injury could result from sex discrimination or race discrimination."
So, being an intersectional feminist means not just caring about gender. Crenshaw's original application of the term specifically addressed racial discrimination; in the decades since its coinage, intersectionalism has also grown to include LGBTQ rights, ableism, and other concerns that undermine the equality of all people. The idea is that systems of oppression all intersect; to consider the issue as a whole, we have to consider all the moving parts that belong to it. It's what contemporary feminism absolutely requires to be effective: As Flavia Dzodan now famously wrote in 2011, "My feminism will be intersectional or it will be bullsh*t."