If something in a lisp program is not a number, string or parenthesis, then it probably is a symbol. It is just a name in a package (which is not entirely unlike a namespace). Symbols may have a global value and/or a lexical value or even no value at all. The value may be a simple value like a number or a string, or a more complex value like a function.
user > (undef 'z) it: null user > 'z it: z user > :d #s( :name "z" :package "user" :symbol z :usage undefined ) user > (var z 123) it: z user > z it: 123