Heat transfer coefficient is how easy the heat passes from one material to
another, between solid and fluid or gas, air. Like it's in the case of heat
transfer through a wall, heat exchanger or convection oven.
Heat transfer coefficient for water is between 500 to 10,000 Watts (meter
square kelvin), air is 10 to 100 watts meter square kelvin.
SI unit of heat transfer coefficient is watts per squared meter kelvin. Other
units are btu per square feet fahrenheit, joule second per square meter kelvin
etc.