Movement of the shoulder blades away from the spine (protraction) or toward it (retraction), as in the top of a push-up versus the top of a row.
Protraction is reaching forward and rounding the upper back slightly; the serratus anterior does it. Retraction is pulling the shoulder blades together; the rhomboids and mid traps do it.
Both are trainable and both are needed. A bench press wants retraction to give the shoulder a stable base; a push-up wants full protraction at the top, which is the rep most clients skip by stopping with locked elbows and a flat upper back.
A quick test: have the client do a push-up plus, pushing the upper back to the ceiling at lockout for a count of one. If the shoulder blades barely move, the serratus needs work, and three sets of 10 push-up plus movements twice a week fixes most of it inside a month.