Lithium Polymer batteries usually come packaged as a pack of more than two individual battery cells. For example, electric RC helicopters use 3-cell packs. Each of these battery cells has a nominal voltage of 3.7v; which means that each cell can operate when it maintains its charge between 3.0 to 4.2v. To go above or below or below this range can damage the cell and render it useless, or worse, become dangerously unstable and explode.
While the battery is in usage, the power drawn out of each cell is not equal. Therefore, at the end of each flight, the cells in the battery will be left out of balance. A non-balancing charger will stop charging the battery pack once the voltage of the overall pack is full without paying any attention to each cell. This causes the battery to be more and more unbalanced with each charge and would also result in a diminished performance of the battery pack.
A balance charger eliminates any unbalanced-cells symptoms of a battery pack by charging each cell individually; making sure that the cell's voltage remain balanced at the end of each charge. In essence, balance charger lengthen the life of the battery back and maximizes its performance as well as keep the pack stable and safe to operate.