Seismic waves are vibrations (waves of energy) generated by earthquakes. They travel through the Earth like a tsunami travels through the ocean, or the sound travels through the air. The time it takes for seismic waves to arrive at seismic observatories allow scientists to locate the precise location of the earthquake that generated them.
There are two different types of seismic waves: body waves and surface waves. As these waves have different physical characteristics, their arrival times at seismic observatories are also used to map the interior of the Earth.
Source: USGS