Preference stars give speed boosts when they match the conditions of a race. A “Firm” preference star will give a speed boost when a track’s weather ends up “Firm” for a given race. Three preference stars give the maximum speed boost for any given condition.

It should be noted that preference stars do not cause speed penalty when the horse is racing in opposite conditions (i.e. racing in a Left turning race with 3 Right preference stars).

Learn more here.