What is meant by Synchronization Rules Editor ?

You can actually use the Synchronization Rules Editor to create custom rules for what gets synched to Azure AD.

Scenario:

Need to deploy Azure AD connect to integrate their on-premise Active Directory domain with Azure AD. They need to prevent  users which have a givenName attribute that starts with 'KAU' from being synched to Azure AD.

how ? :    Synchronization Rules Editor 

reference:

https://docs.microsoft.com/en-us/azure/active-directory/hybrid/how-to-connect-sync-change-the-configuration