i saw a symbol on the top of my post , like infinity symbol and i clicked on it and now my post has a red thing ! What is this and how can i remove it ?

edit
now the red thing has disappeared but what was that ?

You mean this "infinity symbol" (see attachment)?
That's the link to your post.

Edit: if I right click on that, I can "copy link" and then put the link in my comment:
https://www.gog.com/forum/general/what_is_this_1/post1

(attachment 2)

Thanks ! But i stll cant see any point !? Why should i need this ?

Let's say you want to link to another thread, where someone posted something, that you want to share in this thread...then you can simply use "copy link " of that particular post in that other thread (or the same (long) thread) and put that in your comment, so others can follow that link to the post.
It saves you from copying and pasting a whole comment.

Example: https://www.gog.com/forum/general/gog_games_with_splitscreensame_screenhot_seat_multiplayer/post1

Edit: wording

You're welcome!
Nothing to be scared about :)
That feature can be quite useful, actually.