I assume that at the time of writing, most readers would have been familiar with the actual game show and the fact that the host always opens a door with a goat?
https://en.wikipedia.org/wiki/Monty_Hall#Monty_Hall_Problem
(But vos Savant stated in later columns that most of the critical responses she received assumed that the host was obligated to offer the switch, so they were genuinely confused by the paradox.)