Microsoft has provided a variety of directory synchronization tools to customers over the years, most recently Azure AD Connect (AADConnect). Previously, customers could choose from DirSync or Azure AD Sync (AADSync) as well.
Development of DirSync and AADSync ended long ago, with the announcement in April 2016 that both tools were now deprecated. One year of further support was provided, and that ends on April 13th, 2017. Although the tools won't stop working (at least not yet, but it will happen one day), you will no longer be able to get support from Microsoft if you are having directory synchronization issues.
As a side note, customers using AADConnect who have not enabled automatic upgrade, and have not been manually upgrading, will also become unsupported if their AADConnect build number is earlier than 1.1.
For more information about your upgrade options, refer to these articles that Microsoft has published: