These keys are seen in many places in St. Peters as one of the symbols of the Papacy.
The keys are symbolic of Christ giving Peter the keys of his kingdom as noted in Matthew 16:19, where He says to Peter:

"I will give you the keys of the kingdom of heaven;
whatever you bind on earth will be bound in heaven, and whatever you loose on earth will be loosed in heaven."

Whenever we see a religious figure in a painting who is holding keys or has them nearby, we know that is a depiction of Peter,
who was the first Pope of the Catholic Church, (AD 32-67).