What is Some in OCaml

This Ocaml code traverses a list and outputs the last element.

I dont understand the second condition where we output Some x

let rec last = function | [] -> None | x::[] -> Some x| _ :: t -> last t ;;

• So if the list is empty we return null.
• If x is the last element we return Some x (* what is Some x in this context? *)
• If x is not the last element we go further in the list.