The concept of overall coefficient of heat transfer is used in case of heat transfer by

It is used in calculating the heat transfer, typically by convection or phase transition between a fluid and a solid.
The heat transfer coefficient has SI units in watts per squared meter kelvin: $$\frac{{\text{W}}}{{{{\text{m}}^2}{\text{K}}}}$$ .
The heat transfer coefficient is the reciprocal of thermal insulance.