Your heart is made up of different chambers. The heart is made up of muscle tissue. When the chambers expand, they pull in blood and when they contract they push blood out. This is what causes the blood to circulate through the body, get oxygenated, etc. The pumping sound is the heartbeat.
What causes the heart muscles to expand and contract? Very tiny electrical signals are sent from the brain and delivered to the heart via the nerves. Various medical conditions cause the nerves to deliver these electrical signals at wrong times. This causes the heart to stutter; it attempts to push blood out before it has fully pulled it in from the other side. No proper circulation to organs and the brains causes blackouts and eventual death.
For such cases, doctors implant a small device called a pacemaker into the nervous system which deliver the properly timed electrical signals to the heart and makes it beat in a regular manner.