The harmonic series is an acoustical phenomenon that explains the timbre of instruments among other things. When we listen to a sound we hear a series of sounds called the harmonics series. This is the harmonic series based on C2:
This video shows the frequency analysis of the note C2 played by a piano in real time. Each peak is an harmonic in the series:
The video was made with Adobe Audition and Quicktime player. The piano sound is from Garritan's Steinway collection.