Enabling notes for tasks gives you a collapsable/expandable note area right underneath the task title.

When the notes strategy is on you will see a note icon on the right of the task when hovering over it. 

Clicking the note icon expands the task to show a note area underneath the title. Clicking the note icon again collapses/hides the note.

You can add as much text to the note as you like. 

Whenever a task has a note that contains text, the note icon is colored. 

You can also add a note to a task by using the "--" shortcut that you type in the title while creating a task.

To see if a task has a note, without hovering over it, you can activate the "Notes icon indicator" option by going to ☰ → Settings → Tasks & Projects → Notes icon indicator.

The text in the note can be formatted in a variety of ways. Highlight the text you want to change or use the formatting commands first and then start typing. 

For the list of shortcuts, please check the How to format text in notes article.