Reynolds number is a dimensionless number used to define the type of the fluid flow. we have a lot of other dimensionless numbers, such as Rayleigh, Mach, Prandtl etc.. each help us to define the governing physical phenomena dominating a certain problem, inertial vs viscous forces in case of reynolds number.
Example: let say you want to design a new ship prow. instead of building a full size one and test it you can build a much smaller one, test it in your lab and by comparing or adjusting Reynolds numbers (and other nondimensional numbers) you can predict the behavior of a full size prow (e.g.drag forces)
you can read about "Buckingham π theorem" for deeper understanding
