Unity and resistance are what the fist represented in 1917, when it was first employed by the Industrial Workers of the World, a union organization founded by socialists. And in the 1940s, when it stood for various nations' communist party organizations.
That's also what it meant when it was revived in the 1960s, appearing as a symbol for the SDS, as well as anti-war and feminist movements. It was the basis for the black-power salute given by John Carlos and Tommie Smith at the 1968 Mexico City Olympics. And today, it's the symbol for the Progressive Labor Party (pictured), a political outfit whose website says it "fights to smash capitalism."
It goes deeper than you can ever know.