An angel comes down from heaven with the key to the abyss and a great chain in his hand. He binds Satan and throws him into the abyss for a thousand years, to keep him from deceiving people yet to be born. When the thousand years are ended, the devil AKA Satan is released for a short time.
The millennial reign of Christ is marked by happiness and glory: Martyrs for Christ come to life in the "first resurrection," and they join the rest of God's people whom Christ has brought into the kingdom. All of these reign with Christ for a thousand years -- while Satan is bound in the abyss!