Either you need to lock the cursor to one computer, or you're wondering why it's locked.
Scroll Lock Key
Some keyboards come with a scroll lock key (particularly PC keyboards, but not on Mac keyboards). This key is normally found in the top right of the keyboard, between the Print Screenand Pause keys. We use this key by default in Synergy 1 to toggle cursor lock on and off. Pressing this key will lock the cursor to a particular computer, and then pressing it again will release the lock. It's often handy when you're playing a video game, or doing something else in full screen mode. Mac users can customise this key (see below).
Pressed by Accident
Was Synergy 1 working just fine, and now you can't move your computer from one computer to the other? If everything is still connected and looks like it should work, you might have hit the scroll lock key by accident. In this case your cursor will be locked to one of your computers. You can normally solve this by pressing the scroll lock key to release the cursor lock. If that doesn't work, then you may actually have a connection issue or config problem.
Use a Custom Key
You may want to change the default scroll lock key (especially if you're using a Mac keyboard). To do this, click the Configure Server button (on the server computer), then go to the Hotkeys tab. Click the New button on the left to add a new key binding, then press a key or key combo (e.g. Ctrl+Command+L) to choose a key binding (then click OK).
Next, with that key binding selected, click the other New button on the right. You should see a dialog that allows you to customize the new key binding.
Click on the Lock cursor to screen radio button, and make sure that toggle is selected. Next, click OK on the hotkey Configure action dialog window, then click OK on the Server Configuration dialog window. Lastly, click the Apply button on the main window to apply the new configuration.