Tasks are scripts which run at specific intervals. They are useful for performing any maintenance that must be ran on a regular basis. It is important to note however, that they cannot be relied on - though IPS Community Suite provides a way for administrators to run tasks using a cron, this may not be enabled, in which case tasks can only be triggered when users are active on the site - if the site is inactive at the time your task is scheduled to run, it will run at the soonest opportunity after.
You can create tasks in the Plugin Developer Center. When you create a task, a file will automatically be created in the /plugins/<your plugin>/tasks directory for your task. The doc-comments in this file explain how to implement the task.
See the complete Tasks guide for more information on writing your tasks.