PAOK just beat Benfica 2-1 in the Champions League qualifiers. This means that the Portugese club will have to play the Europa League instead, vacating a spot in pot 2 of the Champions League group stage, which Ajax takes over.
The clubs that Ajax avoids thanks to this result are: FC Barcelona, Manchester City, Manchester United, Chelsea, Atletico Madrid, Borussia Dortmund, and Shakhtar Donetsk. These clubs are also in pot 2 and there can be only one club per pot in a single group. If Benfica had won today Ajax would have faced one of those clubs in the group stage.
The clubs in the draw for the Champions League group stage are separated into four pots. The best clubs are, usually, in pot 1 or 2 and 3 and 3 have weaker sides. This is done so that there isn’t one group with only massive clubs and one with very ‘small’ clubs.
Most of the strongest teams will still be able to face Ajax, since they are in pot 1 which consists of: Liverpool, Real Madrid, Juventus, Inter Milan, Bayern Munich, Paris Saint-Germain, Zenit, Porto, and Sevilla