Tuning slide grease is similar to vaseline, but made up of a much thicker consistency.  By using tuning slide grease, brass players are 1) creating a seal between the slides so that air does not leak out of the instrument, 2) providing a barrier between the slides to prevent corrosion and seizure of the moving parts.

Recommended Tuning Slide Greases:  Hetman 8 Premium Slide Grease, Schilke Tuning Slide Grease.