Do It Again is a free and simple program that allows you to make your computer automatically perform a task for you, whenever you want.
If there is something that you do on your computer over and over, and you do it exactly the same way every time (such as backing up your pictures, checking web-based email for new messages, etc.), you can tell Do It Again how to perform that task (by doing it once yourself to create the new task). Then you can run that task whenever you want and Do It Again will automatically click the buttons and press the keyboard keys (in exactly the same way as when you created the task), while you sit back and watch the task being performed on the screen.
This is also called macro or automation software, as it allows you to record a macro, then play it back to automate the actions of that macro.