The first phase is a resting phase, where the heart fills with blood. In the second phase an electrical signal begins in a spot high up in the atria or small upper chambers, called the sino-atrial node. This signal spreads across the muscles of these chambers, making them contract, squeezing blood into the ventricles or lower chambers. Finally waves of electrical activity wash through the ventricles, making them contract and pump blood into the lungs and into the large arteries of the body. So electrical signals start in the node and spread out through a network of conducting fibres across the heart.
Heart block is like a fault in one of the wires, which interrupts the passage of the nerve signal.
This means that one patch of muscle don't receive the signal to contract, or receives it late once the signal has travelled in a roundabout way to avoid the broken circuit. How much this signal blockage affects the heart depends on where it is. If it is close to the node then the signal to contract never reaches the ventricles, and they end up beating totally out of sync with the atria. The ventricles usually end up beating much more slowly, about 40 times a minute. It's a bit like the electrical fault in your home wiring being in the main cable coming in - most of the house is affected.