Sync feature in SharePoint provides two-way between SharePoint libraries and your local computer. Getting “No Apps are installed to open this type of link (grvopen)” error message on clicking the “Sync” button in SharePoint 2013? In some cases, SharePoint 2013 sync button not working?
SharePoint 2013 sync button requires Sync client (such as “SharePoint Workspace”)! You can also use OneDrive for Business (formerly SkyDrive Pro) Sync client tool from https://support.microsoft.com/kb/2903984
It is an Office 2013 application used to sync SharePoint libraries to your computer, pick the right tool for your language, and when prompted provide the product key supplied in the above link. Once installed, you should get sync functionality for your SharePoint sites.