The Fighter is a green, cheap, melee troop that that that sports low health and damage for minimal cost. They are somewhat effective in swarms against certain units.
Fighter's are very weak, surviving only a single shot from a musketeer, and 2 from all other ranged units. Their speed and low cost make up for this more or less as swarming the enemy is their specialty. They can potentially be used as a meatshield, though a Giant would be the better option, provided you have the funds.
Fighter's are best used in situations where players need a few more shots to tank for a unit, or if the player has leftover funds to spare after placing their army.