A callout is a text style that stands out more than the rest of the article. You can also choose an emoji that is relevant to the content of the callout.
To insert a callout
Click inside the article where you'd like the callout to appear
To the left of the flashing cursor will be a grey '+' button - click this
From the menu that appears, click Callout
A grey box will be added where you can add text. When clicked into the callout, a small toolbar will display above it with an option to change the emoji.
To delete a callout, just highlight and delete all the text in it.