A weapon with a telescoping bolt (also known as an overhung bolt) is one with a bolt which telescopes over, that is, wraps around and past, the breech end of the barrel. This feature reduces the required length of a weapon such as a submachine gun significantly, and it allows rifle designs to be balanced around the pistol grip in a way that gives "pointability" similar to a pistol's.
While it would be simpler and easier to shorten the bolt to fit completely behind the breech, the bolt must have a certain amount of mass in order to operate reliably with a given caliber. The telescoping bolt moves some of that mass forward of the bolt face, resulting in a bolt which is longer overall, but is shorter behind the bolt face.
Though technically a different, distinct concept, nearly all telescoping bolt submachineguns do use an ammunition magazine located in the pistol grip which is used to hold the weapon.
Read more about Telescoping Bolt: History, Comparisons, Examples
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