Meaning of parameters of control_msgs/JointControllerState.msg

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As the name implies (JointControllerState) this message is/may be used to publish the (internal) state of a joint (pid) controller.

As to the fields:

float64 set_point

the current set point (ie: desired state).

float64 process_value

the current value of the process (ie: latest sensor measurement on the controlled value).

float64 process_value_dot

first time-derivative of the process value.

float64 error

Essentially process_value - set_point (for a regular PID implementation).

float64 time_step

time between two consecutive updates/execution of the control law (ie: delta-t).

float64 command

current output of the controller.

As an example: see the JointVelocityController in ros_control.

Finally: it might be nice to contribute some comments to the message definition file in ros-controls/control_msgs if you now understand what the purpose of these fields is.

That would help future readers and users of the message avoid questions such as the one you posted.