Partial reps performed only in the stretched half of the range, typically added at the end of a set once full reps have failed.
On a lat pulldown that is the top half; on a preacher curl the bottom half; on a lateral raise the first 30 degrees. The claim, supported by a run of recent studies, is that training in the lengthened position produces more growth than the shortened half, so the partials that are worth doing are the ones most people skip.
The usual application: take a set to failure with full reps, then continue with partials in the stretched range until those fail too. Ten full reps then six partials on a lateral raise is a normal outcome.
Treat it as an intensity technique, on the last set of an isolation exercise, once or twice per muscle per week. Doing it on every set buries the client in fatigue for a modest return.