"The Mouth of Truth (Italian: Bocca della Verità ) is a marble mask in Rome, Italy, which stands against the left wall of the portico of the Santa Maria in Cosmedin church, at the Piazza della Bocca della Verità, the site of the ancient Forum Boarium (the ancient cattle market). It attracts visitors who audaciously stick their hand in the mouth." Source: Wikipedia.
The massive marble mask weighs about 1300 kg and probably depicts the face of the sea titan god Oceanus. The eyes, nostrils and mouth are open. Historians aren't quite certain what the original purpose of the disc was. It was possibly used as a drain cover in the nearby Temple of Hercules Victor, which had an oculus—a round open space in the middle of the roof, similar to that of the Pantheon. Hence, it could rain inside. It is also thought that cattle merchants used it to drain the blood of cattle sacrificed to the god Hercules. In the thirteenth century the disc was probably removed from the temple and placed against the wall of the Santa Maria in Cosmedin. In the seventeenth century it eventually moved to its current location inside the portico of the church."
Tourists stick their hands in the mask's mouth because it has a reputation for being an ancient lie detector:
"Long before the modern lie detector and its harmlessly jittering graphs and wires were invented, the superstitious and untruthful faced a much more severe fate between the jaws of the Bocca della Verità, or Mouth of Truth, an ancient carving which is said to bite the hands off of liars." (Atlas Obscura)
This legend apparently dates back to the Middle Ages, but it was popularized by the 1953 movie Roman Holiday starring Gregory Peck and Audrey Hepburn.