The service record for ldap and kerberos are used to identify LDAP and Kerberos server for a domain in Active Directory. When you configure some authentication method, it might ask you to specify the LDAP or Kerberos Server. The simplest way to achieve this is using nslookup command.
nslookup -type=srv _ldap._tcp.dc._msdcs.
nslookup -type=srv _kerberos._tcp.dc._msdcs.