Estus is the lifeblood of the undead, healing their ailments and giving them a fighting change to actually survive in the harsh world they inhabit. It's also limited, takes a few seconds to have a swig and leaves you completely defenseless whenever you drink it. When an enemy hits you on Dark Souls, it generally hurts a lot. Larger foes in particular have a habit of shaving off about half of your health in one go, making it necessary to immediately heal yourself or face imminent death the next time you let your guard down. To do so, you must wait for a break in the enemies' attack patterns, carefully selecting the best moment to quickly have a swig of Estus and hopefully live to fight another day. Of course this often leads to the enemy executing a hitherto-concealed attack that forgoes everything its done before and completely destroys you long before you've had a chance to heal. Oh well, back down through all of the enemies again to get to the boss and die again!