They're rounds. Stations are offered a lower price in each round which they can accept or reject. If they reject that price, the previous price (which they did accept) will be paid to them if they can't be fit in the TV band. Otherwise, they're out of the auction and placed back in the TV band. There's 52 rounds total, which is based on how the price offered decreases with each round.
After the reverse auction ends, the total amount of money that is required to pay off the broadcasters will be announced and then the forward auction will begin. There has not been a public announcement of when those things will happen yet, though. If the forward auction fails to raise enough money to cover the reverse auction costs, then the TV band is made bigger, and the auction repeats itself with more places to put TV stations so fewer have to be bought and the cost of buying stations overall is lower.
- Trip