The question mark, ?, was a punctuation mark used in many documents on Earth to denote an interrogative.
As fashion for the Doctor Edit
Some incarnations of the Doctor had question marks on their outfits. This was because nobody knew who the Doctor really was since very few people had learned his true name. The Fifth and Sixth Doctors displayed question marks on the collars of their shirts. (TV: Castrovalva, The Twin Dilemma)
The Seventh Doctor wore a jumper covered with the symbol (TV: Time and the Rani) and carried an umbrella with a large red question mark-shaped handle (TV: Delta and the Bannermen) and a business card with a stylized question mark. When asked to sign his name, did so with a question mark. The Seventh Doctor liked using question marks to show that he was a mysterious man and that nobody truly knew him. (TV: Remembrance of the Daleks