What the Reynolds number tells you
The Reynolds number Re is a dimensionless ratio of inertial forces to viscous forces in a flowing fluid. It predicts the flow pattern: below about 2300 the flow is laminar (smooth, layered), above about 4000 it is turbulent (chaotic, mixed), and between 2300 and 4000 it is transitional.
Higher velocity, larger pipe diameter, or lower viscosity all increase Re and push the flow toward turbulence. This matters for engineering design because turbulent flow has higher friction losses but better heat and mass transfer.
Re = v × D / ν
v = velocity (m/s), D = pipe diameter (m), ν = kinematic viscosity (m²/s).