Desktop icons - what's the plus/minus for?

sorry to ask such a simple question but…

I have several icons on my KDE desktop and if I hover over them, they highlight (fine) and also show a plus (+) sign in the top left hand corner of the icon space. This, if clicked on, turns into a minus (-) sign, but doesn’t seem to do anything.

What’s it for?

thanks in advance
Nerderello (Gnome and Ubuntu refugee)

OpenSuse 13.1 KDE 4.11.5

Of course it does something:
It adds/removes that icon to the group of currently selected icons.
It is the same as if you were holding the Ctrl key when clicking on the icon.

You can disable this in dolphin’s settings btw. (General->Show selection marker)

People at kde decided to use the +/- for selecting and deselecting things in the kde enviroment. As an example consider dolphon, the file manager, if you wanted to select several files but could not use drag because there are several files in between them then you just click on the upper left of each file to use the + for selecting them, if you selected an additional file and did not want it in your selection then you clck the - and it’s deselected.

Many thanks for your answers.

It’s obvious, now you’ve explained it.