Sometimes we add more to a post than most people want to read, but put it there because we know it is of value to just a few people. I know I do this often to help beginners needing to see the details and have been looking for a way to keep it in the post, yet make it easier for those reading the post to not have to read the details or even see it take up space on the screen.
If you edit post online there is the menu bar at the top
if you click on the last icon you will get a drop down menu
then if you select
the following HTML will be pasted intro your editor
[details="Summary"] This text will be hidden [/details]
This text will be hidden
To use the working example click on the triangle above.
Much larger example