That’s the Hide Details widget, which lets you keep your post tidy by hiding a lengthy side note (or maybe some ugly code, like dput() output) in a folding section.
You can insert the template for one using the gear menu in the posting box:
(Click the gear and select
Hide Details from the pop up menu)
Or you can do it by hand:
[details="This will be shown next to the arrow"]
This text will be hidden
[/details]
Results in:
This will be shown next to the arrow
This text will be hidden