This is a yellow warning, which means that if you encounter this, it will not prevent you from submitting your fonts.
If you're seeing this warning, it means that paths that make up your fonts are overlapping. This can cause issues when the fonts are rendered on customer devices, may produce results that you're not happy with, and can make the file sizes larger!
The simple fix is to make sure there are no overlapping paths in your fonts.
You can do this easily for your entire font file by selecting all glyphs and using the "Remove Overlap" command in Glyphs App. In the Glyphs Menu, select Filter>Remove Overlap.
However, you may not want to remove overlap to all glyphs in the .glyphs file for a variety of reasons. Instead, you can use the "Remove Overlap" option in the Export dialog. That way, you can work non-destructively, and do not need to differentiate between design file and production file.
Alternative: The dialog option can be overridden with a custom parameter in File > Font Info > Instances. Add a parameter called "Filter", and type "RemoveOverlap" in the Value field.
We know that variable fonts often require paths that overlap or intersect to function properly. Please ignore this warning for variable fonts.