Japanese Lesson #027 discusses katakana and yōon (dakuon/handakuon). It explains that yōon are combining characters that change the sound of katakana when added, turning an "i" sound into a longer "ya/yu/yo" sound for characters in the "ji" line (ja/ju/jo). It provides examples of katakana characters with and without yōon added and encourages practice to master the sounds. The document concludes by noting the next lesson will cover an old Japanese alphabet song.