The Directory Sync tool helps you automate the process for synchronizing your users with Forefront Online Protection for Exchange (FOPE). I've written about it previously here at Directory Synchronization for FOPE, which is simply an introduction to the tool. I want to point out a couple more resources.
At times we get troubleshooting questions for the tool. Troubleshooting the Directory Synchronization Tool 9.1 for FOPE is a nice resource for troubleshooting tips.
We also just released a wiki topic on the Office 365 community wiki, How to Set Up FOPE with Office 365 Using the Directory Synchronization Tool for Populating Users to Enable Directory Based Edge Blocking and Spam Quarantine Access. This topic describes how you can use Dir Sync to set up edge blocking, which can help protect your organization against spam and directory harvesting.