$GENERATE directive is available in multi platform DNS server BIND since 8.2 version. It helps you to create hundreds or thousands of resource records with one line in zone file.
Nice example is to use $GENERATE in order to make PTR records for certain DNS zone.
Let’s assume that subnet 192.168.0.0/24 is reserved for use by a DHCP server pool, in this case reverse lookup (PTR) records should like dhcp-#.yourdomain.com. (where # is the number from 1 to 254) can be created with the following lines in the zone file:
1 IN PTR dhcp-1.yourdomain.com.
2 IN PTR dhcp-2.yourdomain.com.
253 IN PTR dhcp-253.yourdomain.com.
254 IN PTR dhcp-254.yourdomain.com.
or with with single line:
$GENERATE 1-254 $ PTR dhcp-$.yourdomain.com.
Wide explanation of this directive you can find here.
P.S. From Wikipedia for newbies:
A PTR record or pointer record maps an IPv4 address to the canonical name for that host. Setting up a PTR record for a hostname in the in-addr.arpa. domain that corresponds to an IP address implements reverse DNS lookup for that address. For example (at the time of writing), www.icann.net has the IP address 126.96.36.199, but a PTR record maps 188.8.131.52.in-addr.arpa to its canonical name, referrals.icann.org.