Oftentimes a single line is enough of a reminder for what you need to do for a task, but sometimes you will want to track additional notes such as the procedure to follow or even just your personal thoughts and findings.

The TaskHelm editor (the dialog you're looking at now) allows associating a text file with each task.  The files are located in your notes directory (e.g. ~/.task/notes), named using the unique ID of the task.  The task editor automatically saves every few seconds and when you close the editor window.