The terror track is the section of the board depicting the terror level, which reflects the mental state of Arkham's inhabitants. The terror level may be increased by Mythos cards or when too many monsters enter the Outskirts.
Once raised, the terror level never decreases; therefore, investigators should be cautious about allowing it to raise (when given the option).
The terror track is explained on page 20 of the Arkham Horror Rulebook.
For each point the terror level increases, a random Ally is removed from the Ally deck and returned to the box. (This does not affect Allies in play.)
When the terror level reaches three (3), the General Store is closed (i.e., a closed marker is placed on the location, and players may no longer visit that location for the remainder of the game).
Where the terror level reaches six (6), the Curiositie Shoppe is closed.
When the terror level reaches nine (9), Ye Olde Magick Shoppe is closed.
When the terror level reaches ten (10), the monster limit is ignored — there is no longer a limit to the number of monsters that may appear in Arkham.