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. The Fourth, Fifth and Sixth Doctors displayed question marks on the collars of their shirts. (TV: Castrovalva, The Twin Dilemma) In an encounter with the Fifth Doctor, the Tenth Doctor gleefully noted his former self's question marks. (TV: Time Crash)
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, he did so with a question mark. (TV: Remembrance of the Daleks)
Several Alternate versions of the Eighth Doctor wore question mark clothing. The Eighth Doctor that married Grace Holloway wore question mark underpants. Joe Smith, a version of the Doctor who worked as a private investigator, wore a question mark tie. (COMIC: The Glorious Dead)