LoggerHead Lite is a an automated combat log enabler.
The "Lite" means it only cares about when you are in a raid or mythic keystone dungeon and is greatly simplified. For classic, heroic dungeons are also included.
/lh for options. Only instances you've entered will show. The first time you enter a raid or start a mythic keystone dungeon, you'll get a popup asking if you'd like to log that instance/difficulty.
/log to manually toggle logging.
If you want to clear out old entries, you can reset the database in Profiles.