A Freezeout Poker tournaments means that a player can't add any more chips to his stack apart from those he wins. Most online poker tournaments run along these lines, especially the major poker tournaments such as the World Poker Tour and the World Series of Poker.
If an online poker tournament is not a freezeout one it means the player can add to his chip stack via a re-buy or add-on. In any event, one can only rebuy or add-on during a specified period after which the tournament reverts to being a freezeout one.
All online poker sites will indicate if the tournament is freezeout or not so look out for it.
Michael Orwell - Editorial Board