It's a measure of the ability of automation to replace human labor. Let's say that, as a middle-class American, I consume enough to employ ten manual laborers

It's a measure of the ability of automation to replace human labor. Let's say that, as a middle-class American, I consume enough to employ ten manual laborers full-time. If we have ten robots for every person, then everyone can live like me and no one has to do the physical labor.