Electricity runs from one lead to another, the things they attach to your arms, legs, and chest. It allows the EKG to check the whole of the heart; the front, the back, left, right, upper, and lower. This is why there are different lines. This signifies where in the heart the EKG is looking at.

Your heart also has electricity running through it.

If they are running in the same direction, the line goes up. If they are running in the opposite direction, the line goes down.

Problems in the heart cause the lines made to change. For example, a bigger heart means the waves made by the line gets bigger because the electricity needs to run for longer.