L'tak Terai was the Vulcan term for a learning disability which included spatial and order dysphasia, affecting learning, reading, and writing. It was much like the Human condition known as dyslexia. The condition was fairly common on Earth, but not so much on Vulcan.
Spock had the condition as a child, having inherited it from his mother, Amanda Grayson. His father, Sarek, considered it a temporary complication remedied at the Vulcan Learning Center, but Amanda disagreed, feeling that Spock lacked support because no one wanted to help a half-Vulcan. She started reading to Spock from Alice Through the Looking Glass, which she described as a story about how to survive when up is down and left is right. (DIS: "Light and Shadows")
While "Light and Shadows" presented L'tak Terai as analogous to but distinct from dyslexia, in "Perpetual Infinity" Gabrielle Burnham specifically called his condition dyslexia.