I am using Ambari-188.8.131.52.
The statement: When you make changes to Rack ID assignment in Ambari, this mappings file will be updated when you push out the HDFS configuration.
Please explain the phrase "push out the HDFS configuration". How do you push out HDFS configuration?
here "push out the HDFS configuration" means when hdfs configration takes into effect, when you try the same , you will see hdfs will be asked to restart , and when you restart the hdfs , rack awareness related configuration comes into effect
After you update the rack in ambari for any host. HDFS service in ambari will show restart indicators, clicking restart means the config files will be updated and namenode,datanodes etc to be restarted(which can also be called as Pushing out configuration).
This page should provide most of your answers http://docs.hortonworks.com/HDPDocuments/Ambari-184.108.40.206/bk_Ambari_Users_Guide/content/ch03s11.html
Let me know if you have further questions on this specific topic.