It’s a Halloween-colored, non-capitalist market system that involves democratic control over the means of production. Many leftists see it as a middle ground between capitalism and democratic production-for-need socialism, since competition for profit is still in effect but workers have full control over their workplaces. Basically an economic-political system where every enterprise is run like a worker cooperative, with capital subordinated to labor (rather than vice-versa like in capitalism). I’m not personally a mutualist (like most leftists, I don’t think it goes far enough), but I definitely have a certain degree of respect for the ideology. As long as capitalism is around, mutualism will be an aspect of the anti-capitalist left; I’ve said before that I’m very okay with trying to get right-libertarians and other market fetishists onto the anti-capitalist left through mutualism. I also see it as a better outlet than capitalism to build up societal infrastructure and the productive forces before an implementation of basically-post-scarcity production-for-need socialism, since structural elements of capitalism remain the same (profit and degrees of competition among firms) but the blessings created from surplus labor value don’t go to an isolated owner class and can be more spread out across people and society (thus actively building up the prerequisites and “training” people for for-need socialism through cooperative organization and a more horizontal superstructure). If the argument is that a country needs to go through late capitalism to produce the abundance necessary to adeptly manage a socialist revolution, then I don’t see why it wouldn’t be possible to harness that same productive engine through a less destructive and oppressive system.