Point sources come from discrete pipes flowing into a river for example. Non- point are more diffuse sources that don’t come from one single pipe or outlet.
Point sources are for example water discharging from an industrial plant of some kind or a waste water treatment plant. Non-point sources include run-off from agricultural lands that may wash fertilizer or other chemicals into lakes or rivers – this may occur over thousands of square kilometres.
From an environmental perspective it is usually easier to deal with point source emitters than non- point.
See pic.