A slave (aka secondary) DNS server is one that acquires the definition of a zone by synchronizing with a master DNS server.
There are a few reasons to run slave DNS servers. For example, a slave DNS server can serve as a back up to a master DNS server. However, the expiration time of a zone should be set appropriately for a master DNS server to be restored before a slave DNS server stops responding due to the expiration parameter in the zone file.
Other reasons to set up slave DNS servers include off loading traffic from the master DNS server.