A spark plug is used as a source of ignition, as the "spark" in its name implies. It is a key component of internal combustion engines and its primary function is to ignite a fuel/air mixture within the combustion chamber of a car, or other system. Figure 1 shows plugs, the spark travels across the gap on the left side of the spark plug.
The voltage in the ignition system must be high enough to create a spark across the spark plug gap (see figure 2), much like a bolt of lightning traveling from the clouds to the Earth. These voltages can be anywhere from 40 kV to 100 kV, in other words high voltage.
Spark plugs are an important part of gasoline engines, but not a part of diesel engines, since diesel is ignited by compression ignition instead. In figure 2 below note that the timing of when the spark plug fires is very important. The combustion needs to happen at a precise time, combusting too early (usually because of ignition from compression) leads to engine knocking.