The problem quickly and available options to fix it
https://blenderartists.org/t/distorted-faces-with-smooth-shading/1102261/7
The cause of the problem in more detail
https://blenderartists.org/t/new-modeler-struggling-with-booleans-and-clean-topology/1100924/4
Few things about posting:
Don’t waste everyone’s time when describing the problem. The problem description alone rarely gives answers, only a closer inspection and verifying the cause of the problem does that. The fastest and best way to post is to always include:
- A full interface screenshots you use to help you write the description
- An example .blend, which others use for troubleshooting and providing you visuals when they answer
- Mention if you use 2.8, in case you skip the screenshots
Don’t write text into images. You’re not writing english correctly for one, and people can’t quote, do a search, or translate the text directly when it’s burned on an image.