Tommy and Billy Maximoff were in many ways the most important characters in the film. Introduced as her own creations in the Disney+ series “WandaVision,” the twin boys were a manifestation of Wanda’s terrible power as the Scarlet Witch. Agatha Harkness told her it was the power of creation: The ability to create life where there was none before. At the end of the series, Wanda made things right and uncreated both her sons and her husband, the Vision. At the very end, while studying the Darkhold, she heard them crying out.
What made their appearance in this film more meaningful is that it revealed Wanda’s secret: she modeled her version of the boys after those that existed in virtually every other multiverse except hers. The multiverse manifesting itself in the dreams of people in other dimensions is one of the things that drove the Scarlet Witch to grief-stricken insanity, along with the inevitably corrupting influence of the Darkhold. She dreams about the boys every night, knowing they were real, and the Darkhold gave her a map on how to steal them from another multiverse.
Beyond that, there’s another level of meaning. Knowing that Billy and Tommy are real and not just beings that Wanda created means that they are very much in play for the slow-boil reveal of the potential Young Avengers. By the time the actors playing Billy and Tommy will be needed for those roles, they’ll be old enough to step into superheroing.