The easiest way I saw it explained was that “rrat” is shortened from “narrative.” So a rrat is just a story or way of explaining certain events that has little to no evidence proving what is stated. Purely speculation. However, its important to note that many people who are against something will falsify evidence to present a rrat (narrative) that is slanderous or outright false because they are against that thing.

The duck is a rrat?

rrat is any crazy theory or headcanon related to Vtuber culture.

EDIT Also it can mean “schizo”. Based on the context.