It's for one specific strain of anarchism: Anarcho-communism (anarcho-syndicalism uses these colors, too, but it's usually used for AnCom).

There are several designs for the different kinds of anarchism. Usually, it's diagonal-black and some other color (e.g.: violet: anarcho-feminism, green: eco-anarchism, ...).

Yellow doesn't count, since ancaps aren't anarchist.

Edit: typo

Doesn't look like an official symbol, and it's also used for signaling. Libertarian socialism?

Cool quote about the black flag, another traditional anarchist flag:

The black flag is the negation of all flags. It is a negation of nationhood ... Black is a mood of anger and outrage at all the hideous crimes against humanity perpetrated in the name of allegiance to one state or another ... But black is also beautiful. It is a colour of determination, of resolve, of strength, a colour by which all others are clarified and defined ... So black is negation, is anger, is outrage, is mourning, is beauty, is hope, is the fostering and sheltering of new forms of human life and relationship on and with this earth

Arkhos (as in an-arkhos = anarchy) means "chief" (anarchy = no chief).

Anarchy, loosely, is governance without a figurehead or centralized leadership. I'd argue the stars and stripes doesn't mean "Biden" or even "the executive branch", the union jack doesn't mean "Rishi Sunak" and the tricolor doesn't mean "Macron"

I'd argue you could radically amend the system of government and still keep the flag.

I reject your ~~reality~~ flag and substitute my own.

Whoops

Anarchism isn't the absence of structure, but the presence of a complex web of horizontal organization.