le ojwa le sale metsi

I am assuming that means, something along the lines of 'raw talent', 'untapped talent'?

diane tsa setswana dire: lore le ojwa le sale metsi.

My basic explanation for it:
Not sure what "lore" is but clay is molded and shaped while its still wet. So the purpose of the saying is to encourage a good shaping and molding of a person's nature, while they are still young: good instillation of education, respect, morals, skills, interests, etc.