Stones of Barenziah Quest Markers
The image you see here shows the location of one of the Stones of Barenziah, an item that you must find to complete Skyrim's "No Stone Unturned" quest.
"No Stone Unturned" is, without a doubt, one of the more frustrating quests in the game if you try and find the 24 stones without help. The game doesn't tell you where they are so you need to search the entire map for them. Which, as you can imagine, will take you a long, long time.
An extremely handy mod and one we recommend if you're looking to complete this quest.