When you've connected your Samsung Galaxy S2 to a PC or laptop by USB and done all the stuff with either KIES or dragged and dropped all the stuff you need to do you'll want to safely remove your phone from your computer. Usually to do this sort of thing you need to click something on your laptop however with the Galaxy S2 things are easier.
Instead of "safely" disconnecting you need to look at your phone and you will see a screen like the one on the left. If you click "Home" on the phone at this screen it will disconnect you from the PC connection. If you wish to leave the phone to charge you can leave the phone physically connected but outside of the "USB connection mode".