Used as a function word to indicate position in contact with and supported by the top surface of.
This sign is a standalone for the English word "on" for its sake but in practice, it's rarely used in ASL. When using it, one often fingerspells it. E.g. LIGHT+#ON.
Most ASL prepositions are incorporated into verb phrases and classifier phrases.
This fingerspelled loan is also commonly used in some contexts.
Review 101 words. Refresher.