In Chapter Five, Squealer claims that Snowball's heroism is "much exaggerated." Squealer does this to blacken Snowball's reputation after he is driven from the farm by Napoleon, but it is also significant because it foreshadows a number of other events in the story.
First of all, it foreshadows the blaming of Snowball for the destruction of the windmill in Chapter Six. Even though the windmill is destroyed by bad weather, Napoleon instantly blames Snowball and uses this as an opportunity to sentence him to death.
Secondly, it foreshadows the murder of the pigs and the hens in Chapter Eight. In order to justify these extreme acts of violence, for instance, Napoleon accuses these animals of being Snowball's "secret agents," a move which further cements Napoleon's dominance while ensuring that the animals are too afraid to defy him.