Thermochromic substances are materials that change colour depending on the temperature. Thermochromic 3D printing filaments are modified versions of common filament plastics, like ABS. The effect of the modification is that the 3D printed product will change colour when exposed to a temperature change.
Whilst the thermochrome effect can be used for purely aesthetic application (think 'mood rings'), it also has some very useful applications from a functional perspective. One of these potential applications is as an indirect temperature monitoring system to easily visualise temperature changes. The colour change and the temperature that this happens at will depends entirely on how the filament has been manufactured and can be checked on the specific Technical Data Sheet.