Drop shadow is a technique used to add depth and realism to illustrations. It can be used in both the foreground and background of an image, and can be used to create shadows on people, objects, or backgrounds. To use drop shadow in Illustrator, first open the file you want to work with and select it using the selection tool. Next, click on the drop shadow icon located in the bottom left corner of your screen. This will open a dialog box that will allow you to choose how much drop shadow to add. You can either choose a single point of drop shadow or create a series of drops that fall from the top or bottom of an object. Once you have chosen how much dropshadow to add, click on OK and your file will be updated with the new shadows.