This is to say that some of them feed or otherwise gain benefits from the activities of others without depriving these of anything at all. Commensalism is, thus, distinguished from parasitism, whereby some animals feed off the bodies of others, thus depriving these others of something valuable. A form of commensalism is evident in the market place, as when the establishment of one business gives rise to other, adjacent ones that take nothing from the first. Perhaps the best example is concessionaires at sports and entertainment events. Also, if some big firm opens a plant in a neighborhood, other, smaller, firms often start up around it.
Nothing is taken from the big firm in the process.
This commensal relationship may be an alternative to what many see as a the allegedly zero sum relationship of the market place, whereby the gain of some must involve the loss of others. Indeed, a great many people see the market place as a kind of boxing ring so that for one to win another must lose. Maybe even the public goods problem can be understood a bit better by analogy to commensalism. If some folks pay for a service from which others can benefit, if the former were motivated to do so before others, these others can benefit without the former abandoning their pursuits. Thus if it is of value to me to give to a charity, others could just lay back, refusing to contribute or contribute as much as I do.