(Link) The easy way to think about viscosity is about the thickness of a fluid. Water is thin, so it’s low viscosity, while glycerin is thick and has higher viscosity. I liked this definition that “Viscosity is resistance to flow” (reference), that more viscous things require more force to make them flow. So something like glycerin takes more force to make it flow when compared to water. You may see some things measured in centistokes or cS. (1 cS = 1 mPa.s) Silicones are measured this way, especially dimethicone. I’ve worked with 5 cS, 350 cS, and 1000 cS, which you might be able to picture as being as almost as thin as water, thicker than olive oil, and less thick than glycerin respectively.