What is Joint Trajectory ?

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Hello,

I set up a joint_trajectory_controller for my simulated robot on Gazebo.I am trying to understand joint trajectories so I can confirm that my robot moves as I expecting to do.I see in the trajectory_msgs/JointTrajectory Message that we have points. What is these points? Points in where? Later in trajectory_msgs/JointTrajectoryPoint Message I see that points have position, velocity and acceleration. I can understand velocity and acceleration but i can not understand what is position. Position of what? Of my joints? In rad or in deg?