I cannot control my fetch robot

edit

Hi all. I am trying to move a fetch robot go straigh. For example my robot spawn in position (0,0). it cannot go straight to (-3,0) or (-3, -3). i see the code here: https://github.com/fetchrobotics/fetc... I don't understand this code:

    move_goal = MoveBaseGoal()
    move_goal.target_pose.pose.position.x = x
    move_goal.target_pose.pose.position.y = y
    move_goal.target_pose.pose.orientation.z = sin(theta/2.0)
    move_goal.target_pose.pose.orientation.w = cos(theta/2.0)
    move_goal.target_pose.header.frame_id = frame
    move_goal.target_pose.header.stamp = rospy.Time.now()

can anyone explain to me please? sorry my english is not good. thanks all!