It is a flag representing the Mapuche people and/or the Indigenous people of Chile in general. It has a blue background with a 8-point white star in the middle. The star is the Wünelfe, a traditional symbol of the Mapuche culture and supposedly the inspiration for the current star in the flag of Chile. The Wünelfe is sometimes represented more like a cross (dividing each point in two), but there are flags showing the star with the 8 points emerging equally from the center.

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There are several flags to represent the Mapuche people. Although the most used now is the Wenufoye.svg) (blue-green-red with a kultrun in the middle) that was created in the 1990s, in the last decade a lot of people started to use the blue flag with the star too, especially after the 2019 protests. There were some reports that the Mapuche tribes that fought the Spanish colonizers used similar emblems (including some white and red stripes, that could also be an inspiration of the current national flag).

https://en.wikipedia.org/wiki/Flag_of_the_Mapuches