In its most basic sense, multimodality is the mixture of textual, audio, and visual modes in combination with media and materiality to create meaning. The collection of these modes, or elements, contributes to how multimodality affects different situations, or opportunities for increasing an audience's reception of an idea or concept. Everything from the placement of images to the organization of the content creates meaning.
Lutkewitte, Cl. (2013). Multimodal Composition: A Critical Sourcebook. Boston: Bedford/ St. Martin's.
While multimodality as an area of academic study did not gain traction until the twentieth century, all communication, literacy, and composing practices are and always have been multimodal.
Kress, G. (2010). Multimodality: A Social Semiotic Approach to Contemporary Communication. New York: Routledge.