A knock-out takes place when there are too many players to play a single-round Round-Robin.
In a knock-out the players are put into smaller groups and then the winner(s) of that group are combined with winners of other groups to form a new group. The winner(s) of the final group are the winner(s) of the entire tournament.
The number of players that advance is selected by the Tournament Director at the start of the tournament. It might be that only one player from each group advances, or it might be more.
keywords: tournament round robin method organize knock out