The alignment guidelines allow you to easily and quickly adjust all objects. Different guides will automatically appear while you’re moving/dragging an object:
Green guides: Show you the center of each object and each page (horizontally and vertically)
Blue guides: Show you frame guides, like the content frame of a page.
Red guides: Show you how you align one object to another.
If occasionally you don’t want to see any guides just hold the Ctrl key and they will disappear.
As you resize a photo or a shape, it will snap to page guides or objects. Also you can see the snap lines for an object without moving the object, by selecting it, then holding down the mouse button.