A separating funnel is a glass jar like apparatus which has a narrow opening at the bottom and there is a tap fixed at the bottom. So the flow of liquid out of the separating funnel is controlled by the tap opening or closing.
A mixture of immiscible liquids like oil water, petrol water etc. are poured in to the separating funnel, placed vertically. The liquids separate into two different layers with oil floating on top of water. So by opening the tap we collect all the water into one beaker. Then we close the tap. We collect the oil into another beaker.
In between the two we may lose/waste some water and some oil, when collection of water is over and we start collecting the oil.