Apply randomness to the Force Field axis that particles will travel around.
When applying rotational forces to particles, the particles will spin around the Z axis of the Force Field's Transform component by default.
Using rotationRandomness allows each particle to deviate from this default axis by the specified amount. A value of 1 allows each particle to choose a completely random axis to spin around, whereas smaller values will constrain the movement more closely to the default axis.
Additional resources: ParticleSystemForceField.