This is a syringe of 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 HY500 series thermal grease offer better cooling performance with high thermal conductivity graphite and powder. This thermal grease should be used to fill the gaps and expanding cooling area between the component and the heat sink.
It has a thermal conductivity of 1.172 W/(mK), which should be plenty for voltage regulators, MOSFETs, motor drivers, audio amplifiers, etc.