Rumbles are events where players can battle using Rangers of different star levels. The more stars a player puts into their rangers the better they will do in rumbles. Usually, these events have specific requirements that have to be met before you can join them such as having 3-star maxed out characters or just simply being level 30+. You can spend premium currency on tickets to overcome those requirements if need be. At the end of each rumble event, players are awarded prizes based on how well they did. Sometimes you even get crystals that you can use to unlock random rangers!