A reverse auction is a type of auction in which the roles of buyer and seller are reversed. In an ordinary auction (also known as a 'forward auction'), buyers compete to obtain goods or services by offering increasingly higher prices. In the context of the Steem blockchain, reverse auction is how curators compete for the curation rewards on any given post created by authors.
The curation "game" works as a reverse auction. If a vote comes immediately after a post is made, then 99.94% of it will be sent back to the reward pool, after 7.5 minutes 50% and after 15 minutes 0%. Curators must figure out when is best to vote based on how a given article has a chance of becoming popular.