In your zone file, you have lines under the SOA line that are numbers. The first line is the serial number. Usually people use a date and serial number such as
2004122001 and increase the number whenever a change is made.
The second number down is the refresh rate. How often the slave should check for changes. It is usually annotated in seconds, but can have things like 1D for 1 day, 2Mo etc.
The 3rd number down deals with when the slave tries to connect to the master if it can't the first time.
The next number down is how much time before the slave gives up trying to connect to the master
Finally, The last number is how long the slaves info is good for if it can't update with the master.
Many bind examples have default numbers. You do need to up the serial number manually though. Redhat uses something simple like 1 2 3 4 etc, but as I said, most people use a date in there too.