A BGP router with synchronization enabled does not install iBGP learned routes into its routing table if it is not able to validate those routes in its IGP.
The BGP syncronization rule states that if your AS passes traffic from another AS to a third AS, BGP should not advertise a route before all the routers in your AS have learned about the route via IGP. BGP waits until IGP has propagated the route within the AS. Then, BGP advertises the route to external peers.
Taken from BGP case studies:
Can you disable Synchronization?
Yes BGP synchronization can be disabled with the following command.
You can disable synchronization if all the routers in your AS run BGP, or if you do not pass traffic from a different AS through your AS
As of 12.2(8)T, synchronization is disabled by default.
I hope now that when someone asks you “What is BGP Synchronization?” you can answer their question.