A low block is a defensive shape in which a team drops almost every outfield player into their own half, often to the edge of their own box, to deny space in behind and force the opponent to break them down in a crowded area. It trades possession for compactness, usually to protect a lead against a stronger side.
Sitting deep is as old as the game, but Greece's run to the Euro 2004 title is the example most often reached for: a squad with modest individual talent that went the whole tournament conceding almost nothing, built around a settled back line that knew exactly where to stand without the ball. Otto Rehhagel's side beat the host nation, Portugal, in both the opening match and the final.
Chelsea's 2012 Champions League final is the sharper example of it working under real pressure. Away at Bayern Munich's own stadium, Roberto Di Matteo's side spent long spells with almost the whole team behind the ball, survived until a late equaliser from Didier Drogba, and won the penalty shootout that followed.
A low block only works with discipline. Every player has to hold their position and their patience, because one gap anywhere in the line lets the whole shape collapse, and a team using it usually creates very little going the other way.