All the objects (art, text and photos) on your project can be locked and unlocked. When objects are unlocked, they can be moved around, re-sized, rotated, flipped horizontally or vertically, and layered. When an object is locked, it cannot be moved around or re-sized, but it can be given certain effects like a drop shadow and fill color.
The most important thing to remember is that if you can't do something to an object when it's locked, try unlocking it first—and vice verse.
You can tell whether an object is locked or unlocked by first selecting it (click on it once with your mouse) and observing the selection border around the object. If the selection border is a thick pink square, the item is locked. The lock button in the bottom right corner of the toolbox window will show the object is locked.
To lock and unlock an object, first select it, then click the lock button. You can also use the quick keys CTRL + L to lock and unlock an object. For a Mac, the quick keys are Cmd + L.