A Venturi tube is a device used to measure the flow rate of fluids. It consists of a tube that narrows in the middle, creating a change in pressure as the fluid flows through. According to the Bernoulli's principle, as the fluid speeds up in the narrower section, its pressure decreases. This pressure difference can be measured and used to calculate the flow rate. Venturi tubes are commonly found in various applications, including water supply systems and industrial processes, where accurate flow measurement is essential for efficiency and safety.