What makes you tick?
Within each individual muscle cell in our heart there is a difference of charge between the inside and the membrane. Ions start gushing out from the control node in the heart and into the resting cell, making this difference of charge decrease until the cell contracts. A moment later the ions flow back and the cell returns to it's resting state. This will spread like a wave through your heart, and is called a heartbeat!
While this wave of contracting cells is spreading throughout your heart, we can measure the difference of charge between the resting cells and the contracted cells as an electric potential, or voltage if you will. Amplifying this tiny signal and plotting this voltage on paper as it happens is basically the ECG -- the Electrocardiogram.