The Katechon is a biblical concept which has subsequently developed into an important notion of traditional political philosophy.
It is to be found in 2 Thess. 2:3 in an eschatological context : the Christians must not behave as if the Day of the Lord would happen tomorrow, since the Son of Perdition (the Antichrist of 1 and 2 John ) must be revealed before. Paul then adds that the revelation of the Antichrist is conditional upon the removal of 'something/someone which restrains him' and prevents him to be fully manifested.
The interpretation of this passage has raised many problems, since Paul does not speak clearly.
The following interpretations has been given : the katechon is 1) The Name of God (or God's presence) 2) The Holy Spirit 3) The Archangel Michael 4) The Catholic Church (and the perpetual sacrifice of the Eucharist) 5) The Pope 6) The Christian Emperor 7) The Roman Empire (the Holy Roman Empire, not the pagan Roman Empire of the Antichrist) 8) Some more or less important eschatological figure(s) preceding the Antichrist and the end of times (like the two witnesses of Rev.)
Wikipedia is your friend.