As a symbol of peace and security the olive branch is said to date from the biblical legend of the Flood, when the dove returned to Noah's ark with an olive leaf. So Noah knew that the flood had passed away.
For centuries a branch of the olive-tree has been a symbol of peace. In Greece and Rome, those who desired peace used to carry an olive branch in their hand. To hold out the olive branch means to make approaches for peace or a reconciliation.