This flag incorporates the Three Arrows, a common antifascist symbol originally used by the Iron Front, an anti-Nazi paramilitary organisation in interwar Germany.
The specific colour choice was taken from this poster of the Social Democratic Party of Austria ahead of the 1945 election.
The three arrows have been variously interpreted, including as the"political, economic, and physical strength of the working class". Among antifascist symbols, this is my favourite because it turns the same logic of simple and effective symbolism used by the Nazis in the form of their swastika and turns it back on them. Why the arrows point left and down isn't clear but is the most common form.
true, but from what I've seen, since leftists are the ones actually fighting against fascism these days and it's an antifascist symbol, it's been reinterpreted with leftist meanings rather than succdem ones. (also it's not mine)