"Intersectionality" is the observation that your status/lack-of-status in society is the result of not just single systems (black/white, rich/poor, native/immigrant, male/female, hetero/homo, cis/trans) but a combination of all of these.

So a specific person in a specific place is privileged/oppressed as the sum of the advantages/disadvantages provided by each of the identifiable systems. As an example, being a man is usually a net positive in terms of societal privileges, but being black at the same time? Being female is a disadvantage, but if you're also rich, you're still coming out ahead.