The structure word "le" makes a predicate behave as a noun, so that it can be used as an argument of another predicate. The three-place predicate "matma" means "M is the mother of C by father F", so "le matma" means "the mother". Thus "Le matma pa vedma" means "the mother sold (something)", while "Le vedma pa matma" means "the seller was a mother (of someone)".