A girl signs "I love you" (source)
Sign language is a visual form of communication based on hand movements and (to a lesser extent) facial expressions. It is a form of alternative and augmentative communication (AAC) that nonverbal or partially verbal autistic people may use.
Sign language can be taught to very young children in the form of Baby Sign, and it can develop into more nuanced sign language as the child ages (if there is a need). They may also learn other forms of AAC such as typing.
It is recommended that parents/caregivers, and other close loved ones, take sign language classes along with the autistic person so that they can communicate easily.