Our brain has many functions and whether conscious or not, the control tower for humans to live is all in the brain. The mechanics of the brain are quite complex, so let’s take the perspective of a computer to simplify things:
Generally, a computer has basic specifications of processing power, speed and disk space or workable area; our brain is similar in a way that a “high-spec” computer can work quickly such as performing calculations without any delay or hesitation.
In terms of the brain’s “workable area” to that of a computer’s “disk space”, imagine a work desk. When we were young, we had a spacious desk to do work with more than enough area to place numerous documents. As we get older, desks tend to get smaller and smaller, and it becomes more difficult to do work efficiently on such a small desk. Together with a decline in processing speed, it becomes more difficult to find the power to get work done.
This area responsible for the brain’s processing of various information on the “work desk” is the prefrontal cortex in the frontal lobe of the brain. So simply, “brain-fade” means that the function of the prefrontal cortex declines.