Hitting shots that land right on or very near the lines, consistently finding the edges of the court. When a player is painting the lines, they're in the zone. Their precision is at its peak and there's nothing the opponent can do because every shot is landing on the maximum possible angle. Hawk-Eye replays showing the ball clipping the line by millimeters? That's painting.
"She's painting the lines today. Three winners in a row, all landing within an inch of the sideline. Unstoppable."