Skeleton[n] represents a sequence of n omitted elements in an expression printed with Short or Shallow.
The standard print form for Skeleton is n.
In StandardForm, Skeleton is by default printed using \[LeftSkeleton] and \[RightSkeleton] characters.
You can reset the print form of Skeleton.
n indicates the presence of missing information, and so generates an error if you try to interpret it as Mathematica kernel input.
See also: Short, StringSkeleton, Shallow, TotalWidth.