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!
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 tunings. Most slack-key tunings can be achieved by starting with a guitar in standard tuning and detuning or "slacking" one or more of the strings until the six strings form a single chord, frequently G major.
In the oral-history account, the style originated from Mexican cowboys in the late 19th century. These paniolo (a Hawaiianization of españoles—"Spaniards") provided guitars, taught the Hawaiians the rudiments of playing, and then left, allowing the Hawaiians to develop the style on their own. Musicologists and historians suggest that the story is more complicated, but this is the version that is most often offered by Hawaiian musicians. Slack-key guitar adapted to accompany the rhythms of Hawaiian dancing and the harmonic structures of Hawaiian music. The style of Hawaiian music that was promoted as a matter of national pride under the reign of King David Kalākaua in the late 19th century combined rhythms from traditional dance meters with imported European forms (for example, military marches), and drew its melodies from chant (mele and oli), hula, Christian hymns (hīmeni), and the popular music brought in by the various peoples who came to the Islands: English-speaking North Americans, Mexicans, Portuguese, Filipinos, Puerto Ricans, Tahitians, and Samoans.
The music did not develop a mainland audience during the Hawaiian music craze of the early 20th century, during which Hawaiian music came to be identified outside Islands with the steel guitar and the ukulele...
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."
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.