How does subscriber works?

edit

In the code below, turtlebot is moved using /turtle1/cmd_vel topic and we get current pose using /turtle1/Pose topic. My question is if we see the code in if __name__ == '__main__': block, we initialise node, declare subscriber and call for publisher function i.e. motion(speed,distance). This code works perfectly fine. I would like to know how does it get current pose values all the time, how is this program executed? Is it like prem-emptive scheduler, something like RTOS systems, just a thought. I would like to know the background logic and working here.

#!/usr/bin/env python

#libaries

import rospy
from turtlesim.msg import Pose
from geometry_msgs.msg import Twist
import math
import time
from std_srvs.srv import Empty

#initialize variables
x = 0
y = 0
z = 0
yaw = 0

#call back function for subscriber

def pose_callback(posedata):
    global x,y,z,yaw
    x = posedata.x
    y = posedata.y
    yaw = posedata.theta

    #print("Pose CallBack\nx={}\ny={}\nyaw={}".format(posedata.x,posedata.y,posedata.theta))

# publisher function
def motion(speed,distance):
    vel_msg = Twist()

    # get current location
    x0 = x
    y0 = y

    # set velocities to speed
    vel_msg.linear.x = speed

    distance_moved = 0
    loop_rate = rospy.Rate(10)

    # define publisher
    vel_publisher = rospy.Publisher("/turtle1/cmd_vel",Twist,queue_size = 10)

    while not rospy.is_shutdown():
        rospy.loginfo("Turtlesim Moves Forward")

        #publish topic = writing to variable
        vel_publisher.publish(vel_msg)

        loop_rate.sleep()

        distance_moved = distance_moved + math.sqrt((x-x0)**2 + (y-y0)**2)
        rospy.loginfo(distance_moved)
        print(distance_moved)

        if not(distance_moved<distance):
            print("Reahed")
            break


    vel_msg.linear.x = 0
    vel_msg.linear.y = 0
    vel_publisher.publish(vel_msg)




if __name__ == '__main__':
    try:
        # intialise node, node name
        rospy.init_node("my_turtlesim_pose",anonymous=True)

        #call subscriber - read values
        rospy.Subscriber("/turtle1/pose",Pose,pose_callback)
        time.sleep(2)
        print ('move: ')

        #write values
        motion (5.0, 5.0)

        time.sleep(2)
        print ('start reset: ')
        rospy.wait_for_service('reset')
        reset_turtle = rospy.ServiceProxy('reset', Empty)
        reset_turtle()
        print ('end reset: ')
        rospy.spin()
    except rospy.ROSInterruptException:
        rospy.loginfo("Node Terminated")