I thought as much. It looks like a Stack. It might be a Recent Applications stack.
There is a separator bar to the left of the icon in your photo. The area between it & the Trash is where you can place items like folders or files. The area to the left of the bar is for applications only, the System owns that space while you sort of are allowed into the other space if that makes sense.
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments. After that, you can post your question and our members will help you out.