Slack key is a particular type of playing guitar for Hawaiian music, and sometimes used to accompany hula. Often, the tuning is "open", and the guitar will sound a major chord when the strings are struck (i.e. the notes D-G-D-B-D-G for the common "Taropatch" tuning). The effect of this is that melodies sound particular sweet or "nahenahe". In addition, the thumb plays a base rhythm on the low strings while the other fingers play the melody, and there is a characteristic turn-around or vamp (using a 7th chord usually) and these three things along with the generally gentle nature of playing produce the typical slack key sound. Some famous players where sample tracks are available include Sonny Chillingworth, Keola Beamer, Ledward Ka'apana, Gabby Pahinui (and his sons), and quite a number of others. Googling these or finding them on Pandora should help to understand what "slack key" is... Enjoy!
It's a style of guitar tuning where some of the strings are loosened to make them lower and give the guitar an open tuning. Open tuning means the strings play a full chord when played without pressing on any frets. Standard tuning is not an open tuning and so playing a guitar tuned to standard tuning without pressing on the frets is not a pleasant sound. It's very common in Hawaiian music to play slack key, but can usually only be identified by trained musical ears. Though open tunings are popular around the world, there's a style of play on the slack key guitar that is typical in Hawaiian music and I've only ever heard it called "slack key" in Hawaiian music. I don't know for sure, but I would interpret this sentence as basically saying, "I like Hawaiian guitar music."
Right.... so maybe "I like Hawaiian guitar music" would be a better English translation? Or that explanation could put in the lesson notes?
I'm assuming, or at least really hoping, that it's because the course is still in beta and that someday they will add more thorough notes to more of the lessons and add more skills to the course. Would you happen to know if there's anywhere I can look for information about that?
There are places they could post updates, but they haven't been using them. I can tell you that they ARE already working on the next expansion of the course, but it may take them a while and they seem to be focusing more on adding more audio, Skills, and lessons than on improving the Tips & Notes at this point.
One might imagine that in the final (non-beta) version that these cultural terms would be explained in the lesson, as with other languages.
From Wikipedia: Slack-key guitar (from Hawaiian kī hōʻalu, which means "loosen the [tuning] key" is a fingerstyle genre of guitar music that originated in Hawaii and uses open tuning.