A Canonical Name record (abbreviated as CNAME record) is a type of resource record in the Domain Name System (DNS) used to specify that a domain name is an alias for another domain, the "canonical" domain.
So, you give a 'dnsname' to your config servers. For example mongoCfg1, mongoCfg2 and mongoCfg3.
Those dns you can register on a dns server, or in the hosts files of your mongo-servers:
mongos:~$ cat /etc/hosts
In your mongos config file or startup parameters or even the replication/shard configuration in your mongodb, it is adviced to use those CNAMES.
Example of mongos with cname as configdb parameters:
mongos --configdb mongoCfg1:27019,mongoCfg2:27019,mongoCfg3:27019 --port 27017 --logpath /data/log/log.mongos
In case you need to replace a server, let's say mongoCfg2 needs to be replaced, then you can change the ip for mongoCfg2 in all the hosts files so they will refer to the new config server.