The interface heat transfer coefficient represents the rate of heat transfer across the boundary or interface between two materials or substances.
Depending on which bodies are in contact, we get the law of heat transfer. If two solid bodies - heat conduction between them, and everything is determined by the thermal conductivity of the two bodies. If one of them is liquid or gas, it is necessary to calculate the Nusselt number, through which we find the coefficient of heat transfer. The Nusselt number depends on the flow velocity and geometry. Typical cases are discussed in detail in textbooks.