Hey Amos, we call that Event-cream, it's a product we put on the legs of a horse to make them slippery when jumping natural fences and avoid the risk of trauma :)
Are you talking about the white stuff on the legs??
It's like grease.
It's just so if a horses leg hits a log or something it's easier for the leg to slide/glide over the log instead of the leg getting caught or scratched.