It doesn't. Common lisp is a lisp-2, which means that there are 2 namespaces: one for functions and one for variables. (Technically there are quite a bit more, but that's beside the point.)

Which means that you can do this:

  (let ((- 5)) ; bind the '-' variable to 5
    (-         ; call the '-' function
      -        ; on the '-' variable
      17))     ; and the number 17
               ; this evaluates to -12