what does advertising topics mean?

edit

Rosbag is telling you that in 4 seconds after all the publishers are created, it will start to publish the data. This delay will allow all the XMLRPC handshake to be done by the subscribers. In pseudo code that look like this :

Nodehandle n
pub = n.advertise("topic")
sleep(4)
while(data to publish in bag file) {
 pub.publish(data);
 sleep(nextDataDate-now);
}

In the Talker / Listener sample here) the takler "advertise" roscore that a topic exists...

When Node Talker Advertise that a topic exists, it is equivalent to say : "Hi roscore, I am Node "Talker", and I publish a data called "chatter", if some one is interested (subscribing) for this data, I am reachable on this URI "...