Volume flow rate

Essential knowledge 8.4.A.1.ii gives the reasoning and the derived equation: the rate at which matter flows into a location is proportional to the cross sectional area of the flow and to the speed at which the fluid flows, so

The SI unit is $\text{m}^3/\text{s}$. A tap filling a two litre bottle in ten seconds is running at $2 \times 10^{-4}\ \text{m}^3/\text{s}$.

This is the quantity that the continuity equation holds constant. Writing $A_1 v_1 = A_2 v_2$ is the same statement as saying the volume flow rate does not change along a full pipe. The related mass flow rate is $\rho A v$ in $\text{kg/s}$, and for an incompressible fluid, where $\rho$ never changes, keeping one constant keeps the other constant too.

Two practical notes. The sheet prints $A_1 v_1 = A_2 v_2$ but not $V/t = Av$, so treat the second as something you derive. And $v$ here is an average speed across the cross section; real pipes carry fluid faster at the centre than at the walls, which is a viscosity effect the ideal fluid model deliberately discards.