What does this button do?

I know, silliest question ever but I can’t find a way to phrase it to find an answer through Google and I’ve gone through a few Opensuse intro pages and still can’t find the answer.

What does the button on the top left of this image do? The one just right of the filing cabinet. I can’t find the answer anywhere and when I click on it it doesn’t seem to do anything (though it does shade, as though I’ve activated it in some way).
Sorry for such a ridiculous question but its been bugging me and I figure it’ll be simple enough for anyone to answer.
Thank you!

It pins the application so that it is visible on all your virtual desktops. Of course, this assumes you use virtual desktops. If, like me, you only use one desktop, consider the button decoration.

And it says so when I hover over it: “Op alle bureaubladen”, or in plain English: “On al desktops”.

Thank you both so much.