Primary nameservers and secondary nameservers, in principle is similiar to master and slave, the primary holds the domain's dns record whilst the secondary(s) copies this record as a backup incase the master goes down for some reason or another. This is the reason why two nameservers in different geographical zones are required, if the Primary (Master) goes down the Secondary(Slave) takes over therefore the domain is "a live" at all time, particularly where emailis concerned.
Each domain must have only one primary server (more than one can lead to confusion where the correct primary server is located) and can have a number of secondary servers.
For instance, say you had an account at Xname for the domain name my.com.ru, this domain would reside on their nameservers ns0.xname.org and ns1.xname.org. These nameservers are geographically dispersed so they are considered appropriate for this domain to reside upon.
So to makesure that the domain is kept "a live" incase xnzame's servers went down you look around for another DNS service that offers a secondary server service. Say we go to Afraid.org who does offer this type of service by using their ns2.afraid.org to accepts transfers from ns0.xname.org the primary server to their secondary server. Thus the nameservers in this domain's dns record would read:

my.com.ru. IN NS ns0.xname.org. (primary)
my.com.ru. IN NS ns1.xname.org. (secondary)
my.com.ru. IN NS ns2.afraid.org. (secondary)

each nameserver is in a different geographical zone.