When used with champions with a passive giving health regeneration, like Garen, or Volibear, Regrowth Pendant could allow them to lane for an extremely long time.
This was a very useful item as it allowed your champion to stay in lane for longer periods of time. Many players built it into a Philosopher's Stone for its superior effects for a relatively low cost (365g).
It appears to be a metal pendant holding an emerald. Emeralds are often associated with healing powers.