Outdoor reset control enables heat to flow from heat emitters smoothly, continuously and at the right rate.
An ideal heating system would continually and instantaneously adjust its rate of heat delivery to match the heat loss of the building it serves. The indoor air temperature would remain rock stable. There would be no difference in comfort regardless of outside conditions.
No hydronic heating systems can claim to meet this ideal. Instead interior air temperatures cycle up and down (albeit less than with most forced air systems) as heat is delivered in "spurts" rather than as a smooth continuous process. This is the result of water being supplied to the system's heat emitters at the same (design) temperature regardless of the current heating load. The amount of heat delivered to the building is regulated by starting and stopping flow through the heat emitters. It's like driving a car by repeatedly alternating between full throttle and coasting. It'll get you there, but the ride is not as smooth as it could be.