Scheduled Tasks are rules registered in your database by a plugin, theme, or the base system itself; they are used to automatically execute actions defined in the code every certain amount of time. A good use of these rules is to generate backup files of your site, execute a security scanner, or remove unused elements like drafts.
Note that there are some scheduled tasks (registered by the base system) that can not be removed permanently using this tool, tasks such as the addon update and version checker are required by the site to work correctly.