Hot corners are unique combinations of mouse location, mouse button (left/right), and modifier keys state that let you trigger an action by clicking in a screen corner. They are quite similar to hot keys, but there's one major difference: While hot keys are a means to trigger actions without having to move your hands away from your keyboard, hot corners are most useful while you're mousing around.
To assign a hot corner to an item, select the item in question and switch to the Inspector's "Triggers" section. Then, check the radio button that corresponds to your corner/button combination of choice.
To clear a given hot corner, click on the radio button that represents the previously selected corner once more.