Reference: OpenRAVE::IkSolverBase
Each IK solver is defined on a subset of joints of a Robot specified by the robot's manipulator. Given the position in the 3D workspace that an end effector should go to, an IK solver will find the joint configuration to take that end-effector there. Because it is common for an IK solution to have a null space, the IK solver give functionality to expose the free parameters to move the joints in null space.