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