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I don't understand the orientation part. Can you you help me understan the orientation part. I think the value is in radians but I don't understand how it changes with respect to robot position. Also I don't understand the difference between orientation z and w.

The orientation field is a Quaternion. The x, y and z fields can be confusing, as they aren't as intuitively understandable as typical Euler angles or any other rotation system (in fact, without understanding the math behind Quaternions, it's hard to say what orientation a specific set of values encodes at all). They have mathematical properties that make them very useful for representing rotations in robotics applications (well any application, really).

See also the documentation on nav_msgs/Odometry and REP-105 for more info on the defintion and use of the Odometry message.

PS: unnecessary maybe, but the Odometry message is not specific to ROS Aria.