Hello Arthur,
Sync, short for synchronize, basically has Windows Live Mail check for new messages online for all accounts to update in Windows Live Mail, and to update any changes you made to messages on your computer with any online email accounts (ex: Hotmail) you may have as well.
You can set Windows Live Mail to automatically check for new messages for every how many minutes you specify. Click on Tools, Options, General tab, check the "Check for new messages every" box, type in how many minutes, and click on OK.
Hope this helps,
Shawn