"Front lines" refers to the area or position closest to the action or conflict, typically used in the context of war or battles. It represents the place where the most intense or dangerous part of a struggle is taking place. Full definition
«Primary care is our health system's front line of defense, detection, and treatment,» he said. (sciencedaily.com)
Spending on overseas development, on front line health and education services and on the police is to be protected, all this leaving huge cuts in prospect for everything else. (channel4.com)
That is one of the best front lines in a long time. (justarsenal.com)