Arrival behaviour is an extension of Seek steering. When exhibiting this behaviour boids will modify their movement based on their proximity to their target.
Whilst boids are far away from their target, standard seek steering is applied. However as the boids approach their target and come within a specified arrival distance, the boids begins to slow down smoothly, eventually coming to a stop at the target rather than over-shooting the mark which can often happen with standard seek steering.
This type of movement can be used to smooth out motion of erratic boids as they close in on their target.