Guides are lines that you drag onto the document from the rulers. They help you place and align objects more precisely. You can also use guides to measure the size of page elements, or emulate the folds (visible areas) of web browsers.
To help you align elements, you can snap elements to guides, and snap guides to elements. (Elements must be absolutely positioned in order for the snap feature to work.) You can also lock guides to prevent them from being accidentally moved by another user.