The phin is a 3-stringed lute, and this type of Thai music is called phin prayuk (though in videos, the lead plays an electrified double-necked version of the instrument with more strings). This band comes from a remote village in Thailand & usually plays at outdoor celebrations with a home-made sound system. The sound is authentic country Thai –all their own– though to western ears it sounds like wonderful, trancey psych-rock.
Mind-blowing home video that led the producers to find the band and do this recording.
Article about finding the band.