Syncing allows you to access your Zotero library on any computer with internet access. Zotero syncing has two parts: data syncing and file syncing.
There is no limit to data syncing and a 300MB limit ot file syncing, with the option to purchase more. Zotero can be installed on any computer with Firefox, even without administrator privileges.
In order to sync data you must register for a Zotero account, which is free and only takes a few moments: https://www.zotero.org/user/register
1. In Zotero select Edit > Preferences
2. Open the Sync Tab and enter your Zotero account username and password in the server section:
By default files will be synched using the Zotero File Storage (recommended over WebDav):
Syncs of data and files will occur automatically but can be executed manually by using the Sync icon ( )
in the upper right corner of the Zotero window.