An olive branch is a symbol of peace recognized worldwide.
In Ancient Greece, the olive branch was a symbol of bounty and victory. Its association with plenty comes from the essential character of the olive to the Greek economy - without it, Greek life would have been utterly lessened, and dramatically different. The branch was also awarded to Olympic victors.
In Ancient Rome, it was one of many plans with symbolic significance. As the acanthus plan symbolized order, and the oak branch victory, the olive branch symbolized peace.