A Lotus Flower is a delicacy that numbs the senses. The staff in the Lotus Hotel and Casino use this plant to make their guests stay longer and finally not go away at all. The lotus flower is only mentioned in the film.
In the myth, there was a tribe called the Lotus-eaters, who were addicted to the fruit of the tree. When Odysseus encountered this tribe during one of his travels, some of his men tasted the fruit and never wanted to leave again. Odysseus solved this problem by binding them and dragging them back.
Annabeth Chase, Percy Jackson, and Grover Underwood eat the lotus flower. It must be eaten every once in a while to renew the spell. When the trio of heroes first eat the flower, it makes them act like they're intoxicated.
It is said by the trio that it is the most delicious thing they had ever eaten. This is possibly what caused them to become briefly addicted to it; causing them to eat it and the drug to affect them for a time, before Posiedon saved them and escaped the casino.