The volume of dead space represents the volume of the cell not occupied by the sample. This value must be subtracted from the volume of gas injected into the cell to obtain the quantity of gas adsorbed in your sample. This dead space is usually calculated by injection of an inert gaz such as Helium which does not adsorb on the surface of most samples.
Here is a IUPAC technical document on gaz sorption with more info: Article Physisorption of gases, with special reference to the evalua...