The equation of continuity states that a mass of a fluid cannot be created nor destroyed. If we consider the flow of a given mass of air through an opening which varies in size, the mass has to move through in the same time. In other words, what goes in, goes out. Figure 1.1 visualizes this situation:

Figure 1.1 Velocity and pressure variation in venturi tube 
When we consider air flowing through the venturi, the airflow will be a function of the crossectional diameter, velocity and pressure. The product of these factors have to be the same at all points along the way through the tube. Or: A x V x ρ = constant. This means that calculating these factors at any point will result in the same outcome. If we consider the equation of continuity to be applied at speeds below 300kts (where we won't have to bother about compressibility) density changes are neglible and the equation can be written as: A x V = constant
Let us consider to have a given airmass air passing through (1) with a passageway which is reduced along the way untill it reaches its narrowest point (2). In order to maintain the same mass flow, the air has to travel at a higher speed. Eventually when the passage way increases again (3), the speed of the mass flow reduces again
