Any object that has a temperature greater than absolute zero is constantly emitting electromagnetic waves that travel through space at the speed of light. These electromagnetic waves that produce thermal radiation, one of three ways in which heat transfer can occur between different objects, along with conduction and convection.
The video below explores thermal radiation in detail. It starts by covering electromagnetic waves, the Stefan-Boltzmann law, black bodies and emissive power.
It then covers emissivity, absorptivity, transmissivity and reflectivity, as well as view factors, all key concepts for being able to calculate the radiative heat transfer between two objects.