These days there are plenty of online resources to get an introduction to Sacred Harp singing, but David Warren Steel says simply:
“Sacred Harp singing is a community musical and social event emphasizing participation, not performance, where people sing from a tune book called The Sacred Harp, printed in music notation using four shaped notes.”
Singers sit facing inward in a hollow square, and each individual is invited to take a turn “leading,” i.e. standing in the center, selecting a song, and beating time with the hand. The singing is unaccompanied. It’s an old tradition, but there’s not many “sings” in the Northwest. Lucky for us Tim Eriksen will be leading a Sacred Harp session each day at Voice Works this June.
Here’s Alan Lomax talking about it: