Rho is a Greek letter often used to represent density of some kind in this case its mass density so at any given x,y point this tells us what the mass density is mass density will be some mass in a 2-dimensional world so it's mass per area and if we want particular units for our example- once again, this isn't the only way that this can be conceived of there's other applications, but this is the easiest way for my brain to process it we can imagine this as kilogram per square meter and this right over here is the velocity vector it tells us what is the velocity of the particles of that point so this is kind of saying, "How much particles do you have at a kind of a point?