Vorticity – a rotational flow in the atmosphere. Think of it as an eddy current in a river of air.
Positive vorticity implies upward vertical motion and negative vorticity implies downward vertical motion.
With strong vorticities, these eddy currents are usually seen in weather models as pairs, one positive and one negative. The pairing occurs because of basic physics- the conservation of momentum.
Precipitation (with adequate moisture present) is often associated with rapid changes in vorticity, particularly with these positive-negative pairs.