It isn't a measurement of anything. It's a ratio — a tug-of-war score between the forces that want a flow to mix and the forces that want to keep it orderly.
The Reynolds number shows up everywhere in hydraulics — pipe design, open channels, sediment transport, pump selection — and it gets treated like a single magic threshold: below some number, flow is calm; above it, flow is chaotic. That's roughly true, but the number itself doesn't measure velocity, or pressure, or anything physical you could point a gauge at. It's a dimensionless ratio — inertial forces divided by viscous forces — and which force is winning is exactly what decides whether a flow stays in smooth layers or breaks into chaos.