I think they are telling you that they are considering the bulk, or macroscopic, properties of the fluid, and not the properties of the individual molecules. This is an important assumption in fluid mechanics, known as the Continuum Hypothesis.
At the molecular level, tiny, rapid fluctuations in the fluid's physical properties such as velocity and density occur as the molecules move around, bumping into each other (e.g. Brownian motion). These fluctuations would be impossible to predict and model accurately, so fluid mechanics often treats fluids as continuous media with an average velocity, average viscosity, average pressure, etc, on the assumption that the sum of the fluctuations, across millions or billions of particles, cancel out.
This greatly simplifies most fluid mechanics problems and is a valid assumption for most practical problems.