Movement of the shoulder blades straight up toward the ears (elevation) or straight down away from them (depression).
Elevation is a shrug, driven by the upper traps. Depression is the opposite, driven by the lower traps and the lats, and it is what happens at the bottom of a pull-up when the shoulders travel away from the ears.
The common coaching error is treating elevation as a fault. It is not: the upper traps have to elevate and upwardly rotate the scapula for the arm to go safely overhead, so telling a client to keep the shoulders pinned down during an overhead press works against the joint.
Train both. Shrugs for elevation, and a dead hang or a scapular pull-up for depression, three sets of 8 with a two-second hold at the bottom of the range.