You should be able to hover your over each of the buttons and it should pop up a description of what the button is for. When you hover over this one, it says "Spoiler - Hidden text which users can opt to see".
Its the 3rd button on the bottom row in the pic below. Of course, yours may be in a different position, but it should still be between the paper clip and the "</>" buttons.
I've always called it a "callout" bubble.
![]()
If you don't see those buttons, look all the way to the right at the top of the reply box and you'll see the word "More" with a down arrow/triangle. Click the triangle and you'll see the buttons like the picture above.