A shot hit parallel to and near the sideline, keeping the ball on the same side of the court rather than hitting crosscourt. A riskier shot because the net is higher at the sides and you're hitting over the shorter dimension of the court, but a clean down-the-line winner is one of the most decisive shots in tennis. It changes the geometry of the point instantly.
"Forehand down the line. Painting the sideline. Winner. The opponent was already moving crosscourt."