When you score points, that will go toward filling up the Gold Rush bar. Once the bar fills, you'll enter Gold Rush mode, which spawns a bunch of extra fish and also fills your health. You also will be invincible while the effect lasts, and you can earn points by running into objects that would otherwise do you harm. The exact duration of that effect depends on the variety of shark you are using, and also any accessories equipped. When you are in Gold Rush mode, make sure that you move around quickly and eat everything in sight, while avoiding dead ends and long tunnels where you won't have much opportunity to eat a lot before the effect fades.