Look

Particles have their own axis system. Their Z-axis can be aligned manually. In some cases, this can already be done at the Emitter so that, for example, the Z-axis of the particles will automatically point in the direction of flight. However, this alignment only applies to the first frame where a particle is created at the Emitter. The alignment will then retain this direction and will not be updated automatically. However, this is where this Modifier comes into play, which provides precisely this function.

To illustrate this, you can tink of paper airplanes that are cloned onto the Particle Group using a MoGraph Cloner object in Object mode. The orientation of the particles was defined directly on the Emitter according to the direction of velocity. However, it is clear to see that the planes retain this original orientation during the course of their flight and sometimes fly sideways or even backwards.

The following video shows the same simulation for comparison, but this time with an added Align Modifier that automatically aligns the Z-axis of the particles in the current direction of flight. The planes will now always point with their tip in the direction of flight.