Summary of what you need to know to understand the experiment and the results is in the writeup:

Our knowledge of how heat and electricity interact within materials is rooted in the Seebeck, Peltier, and Thomson effects, all identified during the 1800s.

The Thomson effect causes volumetric heating or cooling when an electric current and a temperature gradient flow in the same direction through a conductor.

Scientists have long theorized that a transverse version of this effect should exist when an electric current, temperature gradient, and magnetic field are applied in orthogonal directions in a conductor.

Remember that heat is just molecular motion/kinetic energy; it’s the relationship between the movement of molecules and electromagnetic radiation that’s being explored here, and there’s only so many ways motion and electron energy states can interact with each other.