Mass flow hypothesis is the theory that translocation of sugars in the phloem is brought about by a continuous flow of water and dissolved sugars between sources and sinks.
(A source is the site of production of sugars, usually leaves, and the sink is the site of their utilization, for example the root system.)
At the source osmotic pressure is high due to the continuous formation of sugars and at the sink osmotic pressure is low as the sugars are used up. Thus water from the xylem enters the phloem at the source and leaves it at the sink, returning to the source via the xylem. This tends to drive the contents of the phloem towards the sink.
This hypothesis has been challenged, notably by those who believe there is a metabolic component, i.e. that active diffusion plays a part in phloem translocation.