It sends out an electrical charge at some set interval -- say, once every second, which would establish the low-end normal heart rate of 60 beats per minute (60 to 80 is a healthy heart rate).
These impulses are the "sparks" that cause the right atrium to contract, starting the whole string of events that gets blood pumping in waves through your body. It's this electrical impulse that sets the rhythm of your heart. Whenever the SA node sends out a charge, your heart beats.

When you need more blood pumping, like when you need more oxygen to climb steps or run a mile, the SA node shortens its electrical-discharge interval. There are actually two pacemakers. The SA node is the primary; the atrioventricular node (AV node), located in a bundle of tissues on the border between the right atrium and the right ventricle, is the secondary.
When the SA node sends out an electrical impulse, the first place it goes is to the AV node.