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 it is, and how well the two sides match, is the fastest quality check you can make on a knife in your hand.
The primary grind has to start somewhere. Ahead of the plunge line the blade is ground thin; behind it the blade is full stock. Plunges can be abrupt and vertical or long and sweeping, and neither is better in use.
What matters is consistency between the two sides.
It is the one line on a blade where a small grinding error is impossible to hide. Hold the knife up to a light and compare the plunge on one side against the other: an obvious mismatch is a rushed grind.
It matters functionally too. Asymmetric plunges mean asymmetric bevels, which makes the edge harder to keep centered over the life of the knife.
Just ahead of the ricasso, and often close to a sharpening choil. The three features are neighbors and are often discussed together.
A plunge set far forward leaves more of the blade at full thickness, which affects how the knife cuts.