An electro-cell is a powerful energy source devised by human scientists. The Decepticon Megatron and his forces broke into the facility where the electro-cells were being held and stole them with the intent to use them for a very innovative purpose (i.e. conquest). Megatron's plans were thwarted by the Autobot Mirage in a ploy that involved his teammates briefly believing him to be a traitor.
It is stated that if the electro-cells were to overload, they could start a chain reaction that very well could destroy the entire planet Earth. Exactly how this would work is unclear.