It’s the promise of a new and better world. You grab onto it, and then you find out it just made you bleed for nothing more than wilted garbage.

More seriously, it’s meant to depict promising a new and beautiful world, and the fist is the standard leftist fist of worker power and solidarity. It was specifically designed for the Socialist Party of France, and the idea was to use a symbol that contrasts against the hammer and sickle of the Communist Party. Then it was picked up by social democrats everywhere, especially in Europe.

Typical for social democrats, their primary concern is trying to tell people “we are not communists”