This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
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 pretty 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")