Definition - What does Bent Sub mean?
A bent sub is a short mechanical component installed between the bottom drill collar and the mud motor. The angular design of the bent sub enables the operator to make steering adjustments to the drill according to the desired drilling direction. The bent sub is therefore one of the components in the drilling rig that enables operators to control the path of the drill.
Trenchlesspedia explains Bent Sub
One end of the bent sub is directly connected to the drill string. When the operator rotates the drill string, the bent sub is rotated with it. The other end of the bent sub is connected to the downhole assembly. As the bent sub rotates, it moves the downhole assembly into the desired position. Thus the direction of drilling can be set by the angular design of the bent sub together with its connection to the drill pipe and the downhole assembly.