Impact (also called 'slam' or 'punch') is the combination of fast transient response and high SPL capability in the 60–120 Hz range. It requires: speakers with high dynamic capability, sufficient amplifier power, and a room that does not smear the transient with excessive decay.
Small monitors struggle with impact because their small woofers cannot move enough air. A subwoofer dramatically improves impact. In mixing, transient shapers can enhance or reduce impact without affecting sustain — adding punch to a weak kick drum or taming an overly aggressive snare.