EQ function compares object identity. It works for symbols and identical objects. It is not suitable for comparing numbers - see EQL and =. Result is true if they are same, otherwise false.

(eq 'moo 'moo) => T
(eq 'moo 'foo) => NIL
(eq 1 1) => T
(eq 1 2) => NIL
(eq 1234567890123456789 1234567890123456789) => NIL

(eq (cons 1 2) (cons 1 2)) => NIL
(let ((x (cons 1 2))) (eq x x)) => T