This series is about when to overcall 1NT (and when not to). There are two different situations that arise at the table, one is where you are in the direct seat (when the opening bid has been on your right) and the other is when you are in the balancing seat (when the opening bid has been on your left and there have been two Passes to you. This quiz focuses on direct action - when the bidding has been opened on your right.
It is customary to play a direct overcall of 1NT as a strong bid - say 15-17 points - the same as an opening bid. This is a dangerous position in which to bid - if you catch partner with nothing you may get Doubled and get socked for a large penalty. Experienced players tread cautiously here and therefore require a decent hand to enter dangerous waters. This is why a direct overcall is strong (and also must contain a decent holding in the opponent's suit).