What are overlay icons?
Overlay icons can be shown in the XnView browser for each thumbnail.
In the following screenshot of the XnView browser, you can see thumbnails with overlay icons:

From left to right the overlay icons mean:
Arrow: The file is a shortcut to an image file.
[EXIF]: Image has EXIF information (Info written by digital camera)
[IPTC]: Image has IPTC information (Info typically entered by author of image)
Rotate symbol: The image is automatically rotated according to the EXIF orientation info.
Small square: For displaying the thumbnail of this image, the embedded JPG thumbnail is used.
Can I hide these overlay icons?
If you do not need the overlayicons and their extra info, you can hide the overlayicons. Open the options dialog (Tools > Options), category "Browser > Thumbnail", and deactivate the checkbox "Show overlay icons (EXIF/IPTC/Shortcut)".
Note:
Part of the above info is taken from thread "Info Icons on Thumbnails"