Psychic paper was a blank, white card that had special properties. When shown to a person, it could usually induce them to see whatever the user wished them to see printed on it. However, it proved useless on geniuses such as William Shakespeare. It was most commonly seen being used by the Doctor in his later incarnations.
Psychic paper could receive messages from beings of sufficient psychic ability, but in the hands of untrained users, the paper was prone to displaying facts they subconsciously wanted the reader to be aware of. This could be embarrassing.
When the intended thought was too unbelievable, the psychic paper would only display squiggly lines.
(Gifset from satisfactual)
Part of a series on the #Tech of Doctor Who