Kool-Aid is a sugary sweet drink mainly for kids here in the US.
It's significance is that when Jim Jones had his people drink poison laced juice to commit suicide it has commonly thought (incorrectly) that it was Kool-Aid. It was another, similar product.
Anyway, since then "to drink the Kool-Aid" has come to mean to accept the poison of whatever anyone is trying to make you take. Whether its religious, political or anything else. Anytime seems cultish to people they'll make jokes about "drinking the Kool-Aid".
edited: I see it was already explained. I typed my comment and then the phone rang. By the time I hit the reply button, ya'll had already solved the riddle.
