Original: Bernini's first work at St. Peter's was to design the baldacchino, a pavilion-like structure  tall and claimed to be the largest piece of bronze in the world, which stands beneath the dome and above the altar.
Simplified: Baldacchino and niches Bernini's first work at St. Peter's was to design the "baldacchino" which is like a tent or "pavilion" above the High Altar.