You're thinking of black-body radiation, the phenomenon where an object emits electromagnetic radiation purely because it has a temperature greater than zero.

There are electric charges inside the object - nuclei and electrons. This creates electric fields. When an object has temperature, its particles move around, which means they undergo acceleration. A charge undergoing acceleration in an electric field scatters that field (which also happens when that field is what caused it to move), which creates propagations in that field - electromagnetic waves - light. The higher the temperature, the higher the energy, and Planck gives us a direct relation between energy and frequency. Higher frequency means shorter wavelength. Sufficiently hot objects will therefore emit visible wavelength, while something as warm as, say, a human body emits lower-energy infrared radiation.