When you evaluate
InputForm[Characters["SOMETHING"]]
Mathematica internally produces the result
InputForm[List["S","O","M","E","T","H","I","N","G"]]
i.e. it's an expression with InputForm
as a head, which contains ListList["S","O","M","E","T","H","I","N","G"]
as its first subexpression. You don't see the InputForm
head when Mathematica displays the expression, because the front end only uses it as a hint as to how the expression should be shown, but it's still there behind the scenes.
Then when you use Drop[..., 1]
, it looks at the expression it's given, picks out the first subexpression, which is List["S","O","M","E","T","H","I","N","G"]
, and discards it. That leaves just InputForm[]
.
To make an analogy: if you evaluated
Drop[List[List["S","O","M","E","T","H","I","N","G"]], 1]
you would understand why you'd get an empty list back, right? It's the same thing going on.