How to use mapping.txt file to check the original codes?

HI, I am newcomer for android-classyshark,it's easy to use. But I am confused about how to user mapping.txt to see my original codes. when the APK file is imported in classyshark,I see the confused name like a.a.a And I click "file" icon,pulling my mapping.txt to classyshark. I didn't see any change. Is my operation correct? Please tell me the right way! Thanks !