As I understand, the term refers to the technique popularised by Hawaiian guitarists using the tuning DADF#AD (or 'Vestapol') tuning, though many techniques can also be reproduces in G open tuning. Characterised by a steady (but not overbearing) bass pattern on the bottom strings, the melody lines tend to mirror the tune of the song, using lots of major 3rd and 6th harmonies up and down the fingerboard. Ry Cooder produced and played on a lovely album of slack-key guitar music in the 70s by Gabby Pahinui and friends which is well worth seeking out. Lovely stuff.