To be more precise a "Freudian slip" is an error in speech, memory, or physical action that is interpreted as occurring due to the interference of an unconscious ("dynamically repressed") subdued wish or internal train of thought. They reveal a "source of ideas outside the speech".
On the surface, slips of the tongue may be viewed as simple mistakes or accidents. In a psychoanalytic sense, however, they are symbols that have much deeper personal significance.
Freud believed that when a person utters something other than what they really intended to say, it is no accident but rather their unconscious thoughts which have broken through the censorship of the mind and emerged into consciousness.
For example, a woman who calls her boyfriend by an ex’s name might be viewed as secretly desiring her ex, while a person who mistakenly refers to President Obama as President Osama might be viewed as harbouring negative feelings toward the President.