Clefs are musical symbols that describe the pitch the written notes are meant to be played. The symbol in the photo (placed on its side) is used for bass and baritone pitches, which are on the lower end of the pitch spectrum. A bass clef is often used for cellos and deeper woodwinds (ie a bass clarinet). Bass clef is also used as the bottom clef in keyboard instruments, corresponding to the lower end of the keys that the staff indicates.
Want to see more clefs? Head over to Chromatik for free bass clefs, sheet music, and tutorials.