This folder contains generated code and debugging symbols. It is required in order to debug .dmp files generated from crashes. As the name suggests, it should not be included into the steam build (that is, people who download your game shouldn’t get these files), however, you should store them somewhere that you can access them later when you need them.
Keep in mind that once you delete them, making another build will not produce identical debugging symbols and will cause you to be unable to debug any .dmp files from previous builds should you ever want to.