A rollback is an operation which restores a database to a prior state. In Growtopia, this affects worlds, player inventories, gem counts, and stats usually in order to revert the effects of a game-breaking bug. Consequentially, any work which players put in during the timeframe which was rolled back is lost.
In compensation for lost work and time, rollbacks are often accompanied with an Apology Weekend, a weekend-long event in which in-app purchase values are increased and players have a chance of getting lucky drops from breaking blocks. Any gem purchases made within the timeframe which gets rolled back are given back to the player at double the value.
The first rollbacks occurred on February 23rd of 2013, and resulted in 24 hours of data to be reverted twice. The second rollback occurred only a few hours after the first, and sent players back to the same place they were rolled back to the first time.
The occurrence happened due to an oversight where store-bought items—which had a rarity of 999 at the time—to recycle at a significantly higher rate than the purchase price. Players caught onto this and took advantage of it to effectively gain an unlimited amount of gems. This completely ruined the game's economy at the time. A rollback was issued to restore the game to a state before the exploit was first discovered, and the exploit itself was fixed by adjusting the store items to not recycle for any gems, though they were changed much later to recycle at 10% of the original price. The shop items were also set to not display a rarity.
The Apology Weekend which followed this ordeal granted players to splice the Rollback Plaque in the duration of the event. It was made by splicing a Cave Background Seed with a Door Seed. After the weekend, the recipe would make the new Happy Joy Plaque instead, making it unobtainable.
A second rollback occurred on October 5th of 2013. Only a few hours of data was lost this time.
The issue which ruined the game this time was with special "Null_seed". How to obtain that? It could've been obtained by using a Harvest Moon Blast. When you used that, there could've been a null tree planted. When harvested, it would drop a seed called Null ****(number), and would recycle for random and abnormal amounts of gems, ranging from millions down to negative millions of gems. The glitchy seeds existed long before they were discovered.
The Rollback Plaque II was available during the second Apology Weekend, and was made by splicing a Cave Background Seed with a Sign Seed. Unlike the previous plaque, this recipe was not reused for another item afterwards, until the Rollback Plaque III came out.
A third rollback occurred on August 9th of 2015. The RAID arrays were fried and left Seth with corrupted and missing data for some worlds. They cannot launch the server with corrupted data, so they decided to rollback to the last daily backup (Which means on 8th of August).  The Rollback Plaque III was available during the third Apology Weekend, which lasted from August 10 to August 11. It could've been made by splicing a Cave Background Seed and a Sign Seed. This item, like the other Rollback Plaques, isn't obtainable anymore.