The Heart of Cybertron is a rare and incredibly potent power-source. It can fuel a warship from one end of the galaxy to the other, or be installed within a Transformer, giving them near invincible strength and destructive abilities.
The Heart of Cybertron was used by Megatron to power his almighty warship, the Nemesis. However, when the Nemesis crashed into Earth, the Heart of Cybertron was lost along with it for 4 million years. In 1985, archaeologists (including Joan) uncovered the Nemesis in South America. Soundwave caught wind of this, reporting the find to Megatron, and the Decepticons traveled to South America to retrieve the Heart. After recovering the device, Megatron had Hook install it within his body, amping Megatron's power to a quasi-invincible level.
To thwart Megatron, Perceptor, Bumblebee and Brawn shrunk themselves down and snuck inside his body after he had passed out (inebriated from too much energon). Once inside his body, the three Autobots worked their way to Megatron's chest and disconnected the Heart. Almost immediately after exiting Megatron's body and returning to their proper size, the Heart began to overheat. Brawn hurled it into space, while Perceptor shot it, minimizing the explosion and destroying the Heart of Cybertron.