E-night is a perk for people staying at a Disney resort. They pay 10 or 12/person at the resort and are given a voucher which can be redeemed at the Magic Kingdom for a bracelet.
With this bracelet guests can stay for a few extra hours at the Magic Kingdom after it closes to the general public. The most popular rides (space, splash, Big thunder mtns are all open, just to name a few)
They tend to put a fairly small cap on sales, so as to allow guests little to no wait on the rides.
This only takes place a few times a month.
It is definately worth the extra $.
Hope this helps.