A graphics issue. This might be caused by many things. There are different things you can try to see if it improves.
Most Common Solutions:
-Look at the settings in Revit. Try different selections to see if it solves your problem (notably deselect hardware acceleration).
-Install the latest Revit Updates.
-Update the drivers for your graphics card.
-Install updates for your operating system.
Note: Sometimes it might be required to restart Revit or restart computer to have an effect
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Please mention Revit version, especially when uploading Revit files.