In a slave economy, the masters own their servants directly.
In a feudal economy, the lords own the land the servants live and work on, and the servants are free... to choose which lord they will serve in exchange for the land to live and work on.... or to not work, and not live, if they prefer.
In a capitalist economy, the capitalists own different kinds of capital besides land and the servants can mix and match whose capital they work on and whose capital they live on and try to pair it up so some of their masters^W lords^W capitalists pay enough for work to cancel out what the others charge to live.
What most people today call "communism" is just state capitalism, where there is only one master/lord/capitalist that owns all the various kinds capital and for whom everyone works and by whom everyone lives: the state.
It's all just degrees of abstraction away from slavery unless you own all the capital you need to live on and work with yourself; and by the time you're there, it's an easy step further to get some poor schmuck to work it for you in exchange for borrowing what he needs to live, and then you are part of the problem yourself, an effective slavemaster with your first servant.