The below map of Bubble Witch Saga 2 shows all the current levels that are available to play in the game. To move forward you need to complete every stage before it. You basically keep moving on a forward path and can go back to play any old objectives you would like to get a better score on. For more info visit the Bubble Witch Saga 2 Wiki.
Stars are more for personal achievement in that they don’t do anything for you and will not unlock any new features or hidden areas. In order to get stars easy try to make consistent combos to fill the boards with bugs, if you can make a move try to keep the combos going. This will eventually evolve into a Hot Pot which will send your score through the roof.
As you move forward thing will get increasingly difficult with old hurdles gaining new upgrade making them bigger and stronger. An important thing to remember is that some levels are rigid so that you can’t beat them on your first try. This is the developers basically creating a situation where they could make some more money. After playing something and really looking out for the right move you will eventually move on past there brick laying.
A good tip to remember is that mobile users can move there clock ahead on their device to recover hearts fast without waiting 30 minutes each for them to recharge. When playing on Facebook changing the time on your PC probably won’t work though there may be other means of doing so.