Question

Why does pressure difference cause liqued to flow .

Answer 1

The line highlighted in blue, is what I don't understand. Why does pressure difference cause liqued to flow .

Answer 2

What do you understand by pressure? It is simply force per unit area. If the pressures at two ends of a water column are different-it means forces at the two ends of the column are different. Hence there exists a net force on the water column. This net force would produce an acceleration and cause movement in the water.