Original: Bernini's first work at St. Peter's was to design the baldacchino, a pavilion-like structure 30 m tall and claimed to be the largest piece of bronze in the world, which stands beneath the dome and above the altar.
Simplified: Bernini's first work at St. Peter's was to design the "baldacchino" which is like a tent or "pavilion" above the High Altar. This amazing thing is 30 metres (98 ft) tall and is probably the largest piece of bronze in the world.