Heat Transfer is exactly what it sounds like: the generation, use, and conversion of thermal energy.
From a high level there are three different modes of heat transfer: conduction, convection, and radiation.
Conduction: Heat transfer that occurs across the medium. Medium can be solid or a fluid.
Convection: Heat transfer that will occur between a surface and a moving fluid when they are at different temperatures.
Radiation: In the absence of intervening medium, there is net heat transfer between two surfaces at different temperatures in the form of electromagnetic waves.
In consumer product design for thermal engineering, engineers mainly focus on conduction and convection heat transfer modes.