They may not be as flashy as a powerful weapon or armour item, but flasks in Path of Exile will literally mean the difference between life and death.
Flasks in Path of Exile are not consumable items. Unlike Scrolls of Wisdom, when you use a Flask, it does not vanish. Instead, it simply consumes a number of flask charges based on the flask you're using, and, once empty, must be refilled before it can be used again. Flasks can come in normal, magic or unique rarity, but not rare. Usually, flasks refill as you slay monsters. This means as long as you're successfully engaging in combat, you should always have a flask or two available for use. But if you get into a spot of trouble and empty your flasks, you'll either have to carefully slay enough monsters to refill them or return to Town. Your flasks also refill when you return to Town, so jumping into a portal is a valid strategy when you're running low on flask charges. Some flasks, such as those with the 'Surgeon's' prefix, can be refilled based on other conditions.
Other magic mods can enhance the amount of life or mana a flask restores, or the duration it lasts, or the number of times it can be used. Some also provide extra utilities, such as removing curses or ailments. Carefully selecting your flasks and which mods they have is just as important as selecting which weapon to wield or passive skills to take. If ever you are struggling to survive, take a look at your flasks and see if improvements can be made. They are often a cheap and effective place to start solving character build problems.