What does it mean to go over top?

The Great War 1914 - 1919 (or World War I) was fought in the trenches. Deep lines of French and British trenches faced the Germans. Each mounted large scale attacks on the other. Usually these took place at dawn - there would be a great barrage of artillery fire then the infantry would go 'over the top' of the trenches. These attacks were pretty suicidal, and the expression seems to have become identified with anything that is extreme.