That's auto-sync. Basically, when that's off, you're restricting any accounts on your device from automatically syncing. If it's on, then your accounts will be able to sync.
I thought it might have been rotation lock, I tried it and it's not.
KRazpopov, on 08 February 2013 - 17:10, said:
That's auto-sync. Basically, when that's off, you're restricting any accounts on your device from automatically syncing. If it's on, then your accounts will be able to sync.