Neurotypical is a shorthand for people who are near the average of mental configurations. Its counter is neurodivergent, which covers people with differences in mental wiring, but not so far as to be a mental illness. The term "normal", while technically accurate, implies we are "abnormal", a term often used in a highly derogatory way, even if it is accurate.

Most of the measures of mental state are a bell curve, centered on "normal". If you're centered, you're neurotypical in that aspect. If you move away, you're into the realm of neurodivergent. If you slide too far, the combined result can become a full blown mental illness.

How the scales line up helps define various disorders and quirks of personality. ADHD is one, Autism another, but they are not alone. Since we (vaguely, as a group) got fed up with being over medicalised, the term neurodivergent became popular. Neurotypical is its obvious counterpart.