This is a 5g syringe of white heatsink compound (aka thermal grease, thermal paste, thermal goo). Use this whenever you’re connecting a component to a heatsink, it’ll really help to dissipate even more potentially damaging heat.

This compound is composed of 50% silicone, 20% carbon, and 30% metal oxide. It has a thermal conductivity of 1.172 W/(mK), which should be plenty for voltage regulators, MOSFETs, motor drivers, audio amplifiers, etc.