Syncing makes the references in your EndNote Desktop and EndNote Online libraries exactly the same.
Having the same EndNote libraries allows you to use your EndNote library anywhere on any device. For example, you could use your EndNote Online library on any device (e.g. via the EndNote Online app on your tablet or smart phone) while out in the field conducting a study, experiment or doing practical work . Then, once you get back to your office or study space, you could use your EndNote Desktop library on your computer/laptop for more functionality.
Please see the EndNote Sync page of the EndNote Desktop guide for more details.
Sync Preferences
Note: To access Sync Preference, go to Edit, Select Preferences, and then click Sync in the list of preferences.
EndNote X7.1 (or later) can synchronize an unlimited number of references and 5,000 groups between an EndNote Online and your EndNote Desktop library. References can also be synchronized to a second desktop computer and/or an EndNote iPad library. For the first synchronization on any device, there are several important considerations.