Company sized assaults in World War 1 were conducted on a narrow front to concentrate firepower against the defenders position, so games are generally played on a 4'x4' (120cm x 120cm) table. The Trenchline System gives you a full set of trenches spanning a 4' (120cm) table for your Great War games that you can deploy and be defended straight away.
The box contains all the pieces necessary to deploy a defensive line used in the Great War supplement.
In The Big Push mission for example you place your trenches across the span of the table, with your troops deployed anywhere from the front of the trenchlines to the back of the table. This allows your infantry to lurk in relative safety whilst your guns and tanks can sit further back.
The two communication trenches (which link your front line to the back of the table) provide you with a way to move your reinforcements into the fight without being shot up!