As part of our DNS check, we request SOA (Start of Authority) records for your domain from each of your name servers. These records each have a serial number used for consistency checks between name servers.
These serial numbers are responsible for ensuring that name servers have the correct information.
You will sometimes see your serial numbers become out of sync whenever you update records within your zone and the changes are transferred from your master name server to the slaves. If your serials do not align after a short while then there might be a problem with the transfer. If you see your serial numbers changing when you have not made any changes to your zone, then you should also investigate matters.
The serial number field of the SOA resource record is defined in
SERIAL The unsigned 32 bit version number of the original copy of
the zone. Zone transfers preserve this value. This value
wraps and should be compared using sequence space
RFC1034 uses the same terminology when defining secondary server zone
consistency procedures. [RFC1982]