What is AAC (Augmentative and Alternative Communication)?

My guest for this Glass Half Full podcast episode is Patrick Regan. Patrick is a young man living in Alaska with SMA (spinal muscle atrophy) who uses AAC to communicate. We've gotten to know each other through online BORP classes -- Tai Chi and Pilates.  Our communication previous to our Zoom podcast recording was through Zoom's chat. Patrick is not able to speak and uses technology to communicate via text or assisted speech. Our podcast recording was the first time I actually saw him live since he usually has a photo of himself in the Zoom window. And he varies the photos so I've seen him dressed up for different themes especially in the Rumba with Tina dance class.  Anyhow, Patrick has proven himself to be quite tech savvy and I wanted to learn more about the AAC (Augmentative and Alternative Communication) he uses.