SharePoint Security Sync auto-syncs Dynamics 365 CRM security privileges with that of SharePoint. Any changes made to the security role of a user in Dynamics 365 CRM will be replicated in SharePoint with the help of this SharePoint Security Sync feature.
For example, here in CRM the user can only see a particular record 'Abby Production House' in Account entity.
With SharePoint Security Sync, the user will have the same privileges in SharePoint also i.e. , the user will only be able to see 'Abby Production House' record of Account entity in SharePoint site.