Silver coins can be used to buy boosters that give you a head start in levels. You can buy these boosters before the start of any level - just tap the booster on the pre-level screen so that a green check mark appears next to it, and you'll have the booster for that level.
The booster options are as follows:
- Bombs - one of these will appear randomly somewhere in the level at the start
- +3 Moves - this increases the total number of moves you have to complete the level by three
- Shuffler - use this to shuffle the position of the balloons at any time