The 12/8 ballad — four slow beats, each divided into three. Fifties doo-wop, soul ballads and blues slow-twelves all ride this; the right hand rolls the chord through all three eighths of every beat while the left hand holds the root for a dotted quarter.
How it sits: the drums play a backbeat in compound time — kick on beats 1 and 3, snare on 2 and 4, hi-hat on every eighth — so the triplet division never stops. The bass plays root on each beat with a short pickup eighth into the next chord. The tempo is given as the quarter note (the unit of at): 84 here means a dotted quarter beat of 56.
Listen for the lilt — three equal eighths, not a dotted-eighth-sixteenth. The snare on 2 and 4 lands on the first of a group of three, never late.
Practice: count "1-trip-let 2-trip-let" and let the right hand keep the triplets while the left hand plays only on the beats.