Softaculous is an auto-installer, or script installer, that allows you to install and manage free web scripts.
Web scripts are programs/software that can enhance or add functionality to your website. Examples include guestbooks, forums, image galleries, and e-commerce systems.
The software is installed in your hosting account as a collection of files and directories. The amount of space required depends on the script, but can range from a few kb's to upward of 50mb. Many scripts also require a database to store additional data. Softaculous handles the setup and configuration for you.