Simplified Common Lisp reference
Symbol class: Mathematics, Arithmetics, Logic and Comparisons
Symbol type: symbol
t => symbol

T symbol denotes true value. There is also ubiquitous T constant which contains T symbol. T is not only true value, all values except NIL are treat as true by comparison functions and control operators. Note that constants cannot be redefined (even locally) so there is no chance to make variable t in same name space with T (the true symbol).

't => T
t => T
(eq 't t) => T
(not t) => NIL
(not nil) => T
(not 234.3) => NIL
Function indexFull documentation for t (HyperSpec)