Secondary/Slave nameservers (or Secondary for short) perform resolution for zones for which they are authoritative. The Secondary DNS server is occasionally referred to as a 'slave' server because it is not usually configured directly, but rather by transferring (downloading) a copy of the zone file from the Primary Master nameserver to keep it's own information up to date.
One of the popular myths about Secondary servers is that they are only queried for host information when the Primary Master name server is busy, not responding or down. The Secondary server does perform this function, but since the resolver software built into most nameservers randomly chooses the nameserver it will query, the Secondary nameserver is usually queried just as often as the Primary nameserver. This is why it is sometimes a good idea to have more than one Secondary server to distribute resolution requests across multiple machines, thereby decreacing the load on each individual machine.
Secondary nameservers perform what are called 'zone transfers' periodically from the Primary nameserver. This file is pulled ONLY if the version of the zone file stored on the Primary is newer than the one on the Secondary server.
The timing between these transfers are determined by the DNS Administrator by configuring the refresh and retry settings in the SOA records contained in the zone file stored on the Primary nameserver.