What is a "war of attrition" and how does this phrase apply to WWI

A prolonged war or period of conflict during which each side seeks to gradually wear out the other by a series of small-scale actions is what a war of attrition is. WWI is basically definition of this, the war was fought in trenches and was very slow due to the fact they only progressed when one of the enemy lines was destroyed and one side cross no man's land and took their trenches