The body says:

  We distinguish two kinds of variables: Bound variables and 
  Free ones. The first kind of variables is called De Bruijn 
  indices: each bound variable is a natural number (starting 
  from zero) that designates the number of binders between 
  itself and a corresponding binder. For example, the term \x - 
  > \y -> x (the “K combinator”) is represented as \_ -> \_ -> 1. 
  (If we were returning y instead of x, we would write 0 
  instead.)

The footnote says:

  Contrary to a De Bruijn index, a De Bruijn level is a natural 
  number indicating the number of binders between the 
  variable’s binder and the term’s root. For example, the term 
  \x -> \y -> x is represented as \_ -> \_ -> 0, where 0 is the 
  De Bruijn level of x. The De Bruijn level of y would be 1 if 
  we used it instead of x.↩