It is a code repository of GANimation, a technology that creates animations that change facial expressions by inputting a single image. Basically, it is a conditional GAN, and it uses FACS (facial action coding system), a methodology to describe the anatomical movements of the face.
According to FACS, our face can make over 10,000 facial expressions using about 43 muscles. Action Unit is a unit of specific facial movement, and it is usually said to create metadata using this AU. In addition to creation, such data can be used as augmentation for face recognition, and there seems to be a variety of uses.