Cache is the saved snapshot of a page. It loads the snapshot without spending server resources and memory. It's being deleted automatically, so you don't need to click the button, it should be clicked after some manipulation in database or some change on the website.

If you use some cache plugin, you should make sure the forum page is excluded from that plugin. Here is the instruction: https://wpforo.com/community/faq/wpforo-and-cache-plugins/

So basically this very question is: On the very first page on the forum in the front end. What for and when and how often should I press in the "link clean forum cache" located at the right almost on the bottom of the page?

Thanks