From Hard Drop Tetris Wiki
The Tetris Effect is a psychological condition that occurs when someone devotes so much time and brainpower to a certain interest that that interest begins to appear in that person's sub-conscience. This effect can occur with just about any activity or video game, but the most notable one is the game Tetris. This is why the Tetris Effect has the name that it does.
When Tetris is played for a prolonged amount of time, the person who played the game begins to see the world as a Tetris game, picturing different real-world objects as Tetriminos, and imagining how they would fit together without actually thinking about it. Descending Tetriminos can be seen in the corner of one's eyes and/or when one closes their eyes, and the game can even be seen in dreams. Some people can actually play the game in their mind with their eyes closed after having played the game for a long time. This is due to the fact that Tetris is a separate form of memory. This can be concluded through tests where subjects with anterograde amnesia are given Tetris games to play, and then dream about it later at night even though they cannot remember playing it.
If you are finding this confusing, think of it like sea legs. After spending a long time on a ship, you still have the feeling you are on it after getting off. This is because your body became used to the rocking motions of the ship. The Tetris Effect also works like that.
The Tetris Effect is actually useful in treating Post-Traumatic Stress Disorder. The mind becomes so focused on the Tetris game that the falling Tetriminos begin to replace flashbacks of the horrific occurrence that caused one to obtain PTSD.