Just like the rest of the body, the heart is made up of cells. The muscle cells in your heart are called myocytes (myo means heart in latin or something, and cytes means cell).
These cells are really special because they have the ability to generate an electrical impulse by pumping electrically charged ions (Sodium is the most importiant ion just in case you were curious, but calcium and pottassium also have a role in conduction) in and out of their cell membranes. This process is called depolarization/repolarization.
During the short time the heart is not beating, the inside of the myocyte is negative and the outside is positive (this state is called 'polarized').
(Depolarization)
When the cell is getting ready to contract, ions that carry a positive electrical charge enter the cell until the inside becomes positively charged and the outside of the cell becomes negatively charged. This is the electrical activity that causes the heart to contract. It is called depolarization because the electric 'poles' of the cell have switched.
Repolarization
Afterwards, the cells return to their resting state, positively-charged ions are transported outside of the cell and the cell becomes 'polarized' with the inside of the cell being negatively charged and the outside of the cell being positively charged. This is called repolarization because the cell is returning again to its resting, 'polarized' state.
As I mentioned earlier, any myocyte in the heart can generate electrical activity strong enough to stimulate the other myocytes to depolarize/repolarize in concert with its own 'beat', This means that any of them can act as a cardiac 'pacer' for the rest of the heart to respond to. '
In a heathly heart, this 'pacer' is usually located in the right upper corner of the heart. This location is called the Sinoatrial node (or just the SA Node) This is where the 'Heartbeat' starts.
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