When you make a post, look above the box where you type in the text.

The far left, there is a B for Bold. But notice, on the right, next to the "strike-thru" button, there is another B? It's not for Bolding.

That is the spoiler tag button. when you push it, it adds [SPOIL ] And [/SPOIL ] without the spaces, and those are spoiler tags. They surround whatever you want hidden under a button.