Master database server
The master database server is the primary server running mySQL. The slave database servers replicate data from this central point. If the master database server fails, then a slave can be promoted to be a new master without a horrendous amount of work. See Master switch for how to do this.
To determine which host is the master for the enwiki cluster, you can run
echo 'echo wfGetLB()->getServerName(0);' | mwscript eval.php enwiki
This is how the sql command works.