The term is used by Rafiki when he tells Simba that Nala is his "betrothed," meaning she's destined to be his mate. However, he doesn't go into detail about the meaning of the word itself.
Simba and Nala's understanding of the term is purely through context and the way Rafiki uses it. They likely know that it means something important regarding their future relationship, but the exact meaning is left somewhat ambiguous.