What Does Capital V Represent in Mass Balance Equations?

[Android17]

Please tell me what capital V and v mean in this equation. And how can we derive the second equation from the first one.

 

[Astronuc]

In the continuity equation, mass in = mass out, otherwise there will be an accumulation of mass in a fixed or control volume.

Thinking about a mass flow rate, dm/dt, which can be given by the product of the density and volumetric flow rate. The volumetric flow rate can be given by a fixed area (in a pipe, for example) and the average/mean velocity, usually described by V.

What is the inverse of density?