Your mesh has a hole somewhere and isn't watertight or there is a face that intersects another depending on how your slicer alerts are set up. Basically it's a warning that something is funny with geometry and might not print properly.
If you don't have access to the design and just have the .stl then your best bet is a repair tool. Azure or Trinckle are my go to quick fixes. Trinckle is handy when troubleshooting models because it shows where in the geometry your errors are. 10/10 free service.
www.trinckle.com