Mouth movement is part of non-manual markers in sign language. It conveys an adjective, adverb, or another descriptive meaning in association with an ASL word.
Bickford, J. Albert and Fraychineaud, Kathy. "Mouth Morphemes in ASL: A closer look." 2006. http://editora-arara-azul.com.br/ebooks/catalogo/3.pdf
By Byron Bridges and Melanie Metzger. This 1996 book including the videos provides information about the non-manual signals in ASL.