Wikipedia had the following to say about a top-level binding on the page for Common Lisp:
In Common Lisp, a special variable which has only a top-level binding behaves just like a global variable in other programming languages. A new value can be stored into it, and that value simply replaces what is in the top-level binding.
A variable with top-level binding appears to be another name for a global variable.
The specific example you provided indicates it is in the outer-most/global scope of the program, as opposed to a smaller, temporary scope which goes away during execution. This variable would be available to any portion of the program assuming it had not been locally or temporarily overridden in the current lexical or dynamic context.