In either case, less toxins means healthier human consumers. Intentional selection would involved people noticing the effects of plants and selecting plants from within a species to have less toxic effects. Inadvertent selection could result from people who survived eating less toxic examples of a species planting more of the seeds from that plant.
Work Step by Step
To get to this answer, try to imagine a plant which is nutritious but partly toxic. Think about why farmers might select less toxic individuals to propagate either by choice or by accident/the farmer surviving.