The smooth motion of the agents is controlled using Perlin noise, which is essentially a grid of random numbers. To figure out their next move, each agent looks up the number at their current position and uses it to decide which direction to go in next. For example, if the number is 0 they move at an angle of 0°, if the number is 0.25 they move at 90°, 0.5 at 180°, etc.

If the values in the field were completely random, the agents would jitter back and forth, constantly changing direction wildly. With Perlin noise, numbers that are close to each other in the grid are similar, so the direction only changes a little bit every step, creating a smooth wandering line.