According to the Doctor, the Antizone is a thing the universe makes wherever the fabric of space and time is threatened. A sort of protective buffer zone, if you like.
The Thirteenth Doctor and her friends reached the Antizone through a portal located in a mirror in Norway. Given that its purpose was to protect the structure of space-time, it was a bleak, scary place, full of strange, deadly creatures.
There, they met Ribbons, a sneaky, deceitful creature. He was untrustworthy but the Doctor and friends had to make a deal with him to pass through to the mirror world and rescue Hanne’s father who was residing there. The Antizone also contained met flesh moths. You can guess what they eat! Flesh! Ribbons carried six-legged rats he used to distract them – handy but gross!