This is about conservation. Fluid cannot just disappear. $$A_1 V_1 = A_2 V_2$$
Most simple fluids (water, air, oil) are —their viscosity stays constant even if you stir them faster. fluid mechanics for dummies pdf
Think of this as "fluid friction." It’s how "thick" or "sticky" a fluid is. For example, honey has high viscosity, while water has low viscosity. This is about conservation
You are an engineering student staring at a textbook full of Greek symbols, or you are a curious learner who just realized that water going down a drain and air flowing over a plane wing follow the same rules. honey has high viscosity
Density = mass ÷ volume.