A hybrid deployment offers organizations the ability to extend the feature-rich experience and administrative control they have with their existing on-premises Microsoft Exchange organization to the cloud. A hybrid deployment provides the seamless look and feel of a single Exchange organization between an on-premises Exchange organization and Exchange Online in Microsoft Office 365. In addition, a hybrid deployment can serve as an intermediate step to moving completely to an Exchange Online organization.
But one of the challenges some customers are concerned about is that this type of deployment requires that some communication take place between Exchange Online and Exchange on-premises. This communication takes place over the Internet and so this traffic must pass through the on-premises company firewall to reach Exchange on-premises.
Read about it on the Exchange blog.