This is a symbol of photo stack, which means you have grouped some of your pictures in a stack, and what you see is the first picture of the group, and others are beneath it. To see all pictures:
1. First switch to Detailed view using View->Details
2. Then choose View-> Expand All stacks.
Now you will be able to see all your pictures beneath the first picture of the stack. You can again collapse the stack using View->Collapse All Stacks
If you don't want your photos to be stacked, and rather be shown as individual items, right click the stack, and choose Stack->Unstack Photos
There is also an Option, Flatten stack, selecting which all photos except for the top photo are deleted from the catalog (although not from your hard drive). To do this, right click the stack, and choose Stack->Flatten stack
Let me know if you have any other query regarding this.