The dimensional control was the control system in a TARDIS which controlled the link between the dimension the interior of the TARDIS was in and the dimension the exterior was in. It therefore meant that the TARDIS could have a larger interior than exterior. If the dimensional controls were damaged the interior would become the size of the exterior because it would not be in its own dimension.
The First Doctor stole the dimensional control of the Monk's TARDIS, reducing the interior dimensions to that of a doll's house, making it impossible for the Monk to get inside. (TV: The Time Meddler)
A dimensional control was also used to operate SIDRATs. The War Chief remotely activated the dimensional control to close in the walls of one SIDRAT to force the Second Doctor, Jamie McCrimmon and Lieutenant Carstairs out of it, but after the Doctor stole the master circuit rods, it returned to normal. (TV: The War Games)