moveit_tutorials | A sphinx-based centralized documentation repo for MoveIt | Robotics library

My research says that a quaternion of form (x, y, z, w) describes rotation in 3D space only. How does the robot know what position to move to if it only gets rotation information?

Correct, a quaternion describes the orientation of a frame only; you also need to specify the position of the frame to have a complete pose. Quaternion is just a different way to describe the orientation of a body, another way is, as you already mentioned, to use Euler Angles (Yaw, Pitch, Roll).

In your case with your pose_goal object, you need to specify the position of the desired/goal robot pose by setting the x, y and z components of pose_goal.position (that's the position of the Robot in 3D space) and then also specify the orientation of the desired/goal robot pose by using the Quaternion notation w, x, y and z to set the components of pose_goal.orientation (note that x, y and z part of the Quaternion is not the same with your position vector, they are different things). Once you define both pose_goal.position and pose_goal.orientation you are done, you have a complete pose that you can send to MoveIt! to plan and execute.

Can I convert from (x, y, z, roll, pitch, yaw) to a quaternion? Or do those describe two different things?

Here, x, y, z, is your position vector which has nothing to do with the orientation (and hence with quaternions), so your question should really be, "Can I convert Euler angles (roll, pitch, yaw) to a quaternion?" and the answer is, yes you can convert Euler angles to a quaternion but it might be tricky. If you can represent the orientation using a quaternion (i.e., if you have this information already) you should use it as quaternions are more numerically robust and they are not suffering from singularities (for example Euler angles could cause a Gimbal lock where under a specific configuration your system loses a degree of freedom).

If you have access to a quaternion, use it and forget Euler angles, else you might want to try converting the Euler angles to a quaternion using the following way using ROS's tf library: