The plunge line is the boundary where the primary grind stops at the rear of the blade, seen as a distinct line running from spine to edge just ahead of the ricasso. How crisp and how even it is tells you a lot about grinding quality.
The primary grind is the taper that takes the blade from full thickness down toward the edge. It has to start somewhere, and the plunge line is that start. Ahead of it the blade is ground thin; behind it the blade is full stock.
Plunges can be vertical and abrupt, or long and sweeping. Neither is better in use. What matters is consistency: the plunge on one side should sit at the same point and the same angle as the plunge on the other.
A crooked plunge is the most visible sign of a rushed grind, because it is the one line on the blade where a small error is impossible to hide. Comparing the two sides of the blade against the light is the fastest quality check you can make on a knife in your hand.
It also matters functionally. An uneven plunge means the two bevels are not symmetrical, which makes the edge harder to keep centered over the life of the knife.