Vibranium is a rare metallic element from the Marvel universe. In Marvel's Captain America: The First Avenger, it is the metal from which Captain America's shield is made. The shield constitutes the entirety of the known supply of vibranium on Earth.
Vibranium is said to be able to absorb most kinds of energy in massive amounts. Consequently, it is virtually impossible to damage. In practice, events in Marvel's The Avengers indicate that it actually reflects energy above an unspecified threshold, resulting in a massive shockwave when it is struck by Thor's hammer.