Biotic factors are the living factors that affect a community. The ones you need to know are: availability of food, new predators arriving, new pathogens, and one species outcompeting another so that the numbers are no longer sufficient to breed.
Each factor changes population size. If the food supply falls, fewer animals survive and the population drops. If a new predator arrives, more prey are eaten, so the prey population falls. A new pathogen can spread through a population and cause disease, killing many individuals. If one species outcompetes another for food or space, the weaker species may fall to numbers too small to find mates and breed, and it may die out locally.
Because species are interdependent, one change can have knock-on effects. If a prey population falls, the predators that rely on it have less food, so their numbers can fall too. You may be given a chart, graph or table and be asked to explain how a biotic factor affected a community.