Answer:
Trench warfare was a tactic mainly used during WW1 and is when both sides dig into the earth and make defensive positions. Trench warfare was not effective and would usually result in stalemates. In trench warfare it could take weeks just for 1 side to gain inches. The main form of attacking in trench warfare was call going "Over the Top" were soldiers would rush over the tops of their trenches and charge into the enemy trenches. "Over the Top" was extremely ineffective as trench defenses were strong against infantry attacks due to the use of machine gun fire and barbed wire.
Example:
Here is an example of a trench system I drew today if you need it or are still confused.