AAC stands for Augmentative and Alternative Communication.
AAC is often used by people who can't speak out loud (verbally) in what are regarded as "words". AAC comes in many forms. It can be pointing to pictures in a book. It can be typing. It can be higher-tech, too, with specialized apps.
AAC is not less good or important than other forms of communication.
Most people use some type of AAC, like nodding or making a thumbs-up, or using emoji. However, AAC used by Disabled people often isn't taken seriously enough.
AAC is one kind of disability accessibility.