They are talking about the opening moves of the war. The German battle plan known as the "Schlieffen Plan" was to sweep thru Belgium and get around the French army for a quick victory before anyone could really react.

Besides getting delayed in Belgium, the plan work pretty fine for a military plan. This opens up the time of the war that is known as "Battle of the Frontiers". A time when the German army was inflicting heavy casualties on the French army that had to keep retreating further into France. The German army seemed unstoppable and the war with France seemed to be a repeat of the successes in the Franco-Prussian war.

This is the good old days for the veterans in the book that have survived that long. It is the thing everyone wants to achieve, the breakthrough that will make the war mobile again. To the soldiers in the trenches is this almost the stuff of legends or heroics.

"Battle of the Frontiers" ended not far from Paris in 1914 when the British managed to get in between the German formations. This made the Germans retreat to regroup and they dug temporary trenches to defend while regrouping. The French and British reached this defensive line and, when they couldn't dislodge the Germans, dug their own trenches. Trench warfare became a thing and the "race to the sea" soon followed. The situation that is described in the start of the book came about not to long after this.

In a time when almost every meter is paid for in blood does the fast advance of 1914 seem like heroics.