what are quick tasks? I have read the documentation, but cant seem to understand

Quick task are related action and created under same main process model. It is applied only for Process backed Record.

In a process-backed record, quick tasks serve as the related actions for your process record.

In most cases though, the primary purpose of a quick task is to display a user input task without the task populating in your 'Tasks' list. For example, say a user wants to edit a record and so they start a related action which kicks off a process. Shortly after, they change their mind and click out of the window without making any changes. They don't want the task to show in their 'Tasks' list, because they don't care about it anymore. To account for this behavior, you can make the user input task a quick task and then set an exception timer to except out of the form and end the process so as to not leave it hanging in memory.

A quick task is an ad-hoc task that once enabled within a process instance can be executed and completed n-number of times throughout the life of the process instance. The quick task is a task that may or may not be completed in your process and usually doesn't impact the overall flow of your business process. The quick task has an assignment but uses it more as a permission of who can execute this ad-hoc task.